The keyboard error (if you only get it while pressing buttons trying to get into the BIOS) is most likely only due to you trying too many keys at bootup. If you also get it while booting without pressing any buttons, then it seems like your internal keyboard controller is broken.

On a related note. Old Compaq computers had the software used to enter BIOS stored on a partition on the harddrive (at the start of the drive space tbp). If the HDD is missing, or the partition deleted you are not able to enter Compaq SETUP (the BIOS). You need the appropriate Setup Diskette (still available on HP support site) for this model, and boot from that in order to be able to reach the BIOS and change settings.

The setup diskette for this PC isn't available anymore on the HP support site.

http://oldcomputer.info/pc/compaq510/index.htm#config

Under Configuration, you'll find the following needed files.:

1) If you have a fresh hard drive, use these disks to re-create a hidden partition. Remember that it needs a hard drive clean of any other partitions and it will take first 3-4MB of hard disk.

2) ES688 (Windows 3.x) - 95 should detect it.
Cirrus CL5425 (Windows 3.x), 16MB patch, ESS DOS drivers in one image and in factory-like form - 95 should detect Cirrus chip without problems.
3) DOS and Win95 patches - needed for applying fixes, mouse drivers and supplementary software

NOTE: the Diag and SETUP Diskette is used for both - BIOS changes AND the recreation of the diagnostic partition (optional).
The Diagnostic partition lets you put the BIOS application on the HDD, enabling you to reach it while pressing F10 at boot, but it is optional, you could also just boot from it whenever you need to change something.

Thanks! But how to use these files?
http://oldcomputer.info/pc/compaq510/diag_setup.7z

Those are .img (image) files that hold the information of the whole floppy. A program like WinImage or DiskWrite can write them onto a floppy disk.

Do not just unzip them and drag the contents to the disk, the disk will not be bootable afterwards, but you want it to be. So choose an image reading/writing application and write the contents of the image file onto floppy and boot it in the Compaq to access the BIOS.

Oh, and the .7z files are 7-Zip files. You either need 7-Zip or another unpacker capable of handling these files to unpack them, but out of them do come the .img files that need to be transfered by an DiskImaging program (not just copied to the disk itself)

Thanks! I'm now on this utility. But the HDD isn't working. It always starts and stops and isn't recognized by the utility.

What's the size of the HDD, is it the original HDD?

If so, and it starts and stops, you could try to re-set it with HDAT2 or other HDD Diag software...

But to be honest, if it is the original HDD, even if you can successfully reset it, it is much likely flawed throughout (bad sectors).

The go-to solution in these cases is to go for an IDE-to-CF or IDE-to-SD adapter and use a Compact Flash Card or SD Card as HDD substitute, respectively.

Even if the old original HDD has 0 bad sectors, I'll guarantee you it won't for long, especially if you now start using it again.

The size of the original HDD is 270 MB.

90% chance it's a goner.
Either replace it with a 200MB-2GB IDE 2.5 inch HDD that works
or go the safe route (adapters for SD or CF).

Either or - to break the BIOS limit of (what I guess would be around 500MB), use Drive Overlay software like EZ-Drive or On-Track

Where I can buy the adapters for SD or CF? How much does they cost?

As SD cards are rather cheap, replacing or getting an SD card (remember, even 2 or 4GB would be enough to replace an old HDD 😀 ) is not the problem, as in much cheaper...

Compact Flash cards (especially the smaller ones with 128-2GB capacity) are not very common anymore, and they tend to cost more compared to SD cards.

A 2-4GB SD card for example would cost me around 6 EUR in Germany, whereas CF cards (of smaller sizes) I would only be able to get online second hand, as newly produced ones (besides being quite costly at 30 bucks +) only come in way to big sizes.

As for the adapters, CF or AD adapter cost about the same, either locally (Europe for me) at around 15 bucks, or imported from China for around 5 bucks incl. shipping (but with at least a few weeks of shipping).

Since I didn't want to wait I got my adapters from a shop in Ireland for about 12 bucks a piece. (over ebay)

All you need then is an SD or CF card, and possibly a Drive Overlay software like OnTrack or EZ-Drive, as the chance of the CF card or the SD card being way to big in terms of capacity is very high.
Your computer's BIOS limitation for Hard Drive capacity is probably 504MB.

jaZz_KCS wrote:

Compact Flash cards (especially the smaller ones with 128-2GB capacity) are not very common anymore, and they tend to cost more compared to SD cards.

You're right. Gratefully, there are SD2CF card adapters. They are essentially the same as SD-IDE adapters.
What I liked about (old) CF cards is their sector size. Their flash cells used 512 Bytes per sector internally.
That's very convenient for older operating systems with FAT16B. It's no big deal, though. In terms of alignment,
tools like GParted or newer FDISKs can be helpful to align FAT to the megabyte boundaries (2MiB is often used).
Also, some odd DOS versions used non-512Byte sectors. Maybe they can be made work natively with 4K sectors ?

jaZz_KCS wrote:

A 2-4GB SD card for example would cost me around 6 EUR in Germany, whereas CF cards (of smaller sizes) I would only be able to get online second hand, as newly produced ones (besides being quite costly at 30 bucks +) only come in way to big sizes.

Here's a trick: Just go and buy an eldery digital camera. They often come with a CF card for free. 😉

Drive Overlay software

.. or a BIOS update, an IDE enhancer card or good ol' XT-IDE Unviversal BIOS. 😁
(If someone wants to go the fancy route, a DOC on an ISA card will also do.)

jaZz_KCS wrote:

As SD cards are rather cheap, replacing or getting an SD card (remember, even 2 or 4GB would be enough to replace an old HDD :) […]
Show full quote

As SD cards are rather cheap, replacing or getting an SD card (remember, even 2 or 4GB would be enough to replace an old HDD 😀 ) is not the problem, as in much cheaper...

Compact Flash cards (especially the smaller ones with 128-2GB capacity) are not very common anymore, and they tend to cost more compared to SD cards.

A 2-4GB SD card for example would cost me around 6 EUR in Germany, whereas CF cards (of smaller sizes) I would only be able to get online second hand, as newly produced ones (besides being quite costly at 30 bucks +) only come in way to big sizes.

As for the adapters, CF or AD adapter cost about the same, either locally (Europe for me) at around 15 bucks, or imported from China for around 5 bucks incl. shipping (but with at least a few weeks of shipping).

Since I didn't want to wait I got my adapters from a shop in Ireland for about 12 bucks a piece. (over ebay)

All you need then is an SD or CF card, and possibly a Drive Overlay software like OnTrack or EZ-Drive, as the chance of the CF card or the SD card being way to big in terms of capacity is very high.
Your computer's BIOS limitation for Hard Drive capacity is probably 504MB.

How many PINs must have the adapter? 44 or 40?

Well there are adapters for 2.5" drives (44pin (2pins for power)) and 3.5" drives (40pin, power extra). Since we're talking about a Presario CDS510 here (which uses 3.5" Slimline Drives iirc) you should go for the 3.5" (40pin) variant.

Someone with more knowledge on these models please confirm this, so that Simone does not buy the wrong adapter. I shall not be held responsible 😜

EDIT: also I don't know whether she needs a male or female adapter, pretty sure about 3.5" (40pin) though, but I never had a Presario 51CDS, so please - someone who has/had - input needed 😀

Thanks! I bought a IDE to SD adapter and now my Compaq presario works perfectly with Windows 95!!! I solved all! Thanks again!
https://imgur.com/a/BoKLk