Hi,
my friend have old s775 computer. I forgot type of MB, i will post name later. Computer is in storage now.
It is MSI board with VIA chipset and two DDR2 slots.
First issue is extreme incompatible RAM. Most DIMMs are full of errors in memtest. Mainly 512MB modules works. 1GB modules are detected as 512MB and 2GB as 1GB. Cant remember if both single and dual banks. Is impossible to get working system with more than 1GB RAM. With 1,5GB or 2GB (half from two 2GB DIMMs) is system unstable or hangs.

But one problem is very strange. With Pentium D processor is impossibe to enter SETUP configuration on system start. With Celeron D it works. Why? BIOS was upgraded and Pentium D is detected right. PS2 keyboard wotking in DOS. Of course RTC memory was cleared.
I never saw same issue.

I can't test system now but i want try it in future.