Before doing anything, make a backup. For an operating system, a full system image is best, using something like Ghost; allowing you to go back to the exact same drive state before you started experimenting.
Normally, if you were dual booting XP and Win7 you'd install XP first. That way Windows 7 doesn't have to create that little system partition as it'll put those files on the XP partition and most importantly, it'll automatically configure the boot manager to have both entries.
If you don't want to reinstall Win7, you could try doing an XP install to another partition (ideally the same drive as Win7) and afterwards you will only be able to boot XP. At this stage you could go all ninja or simply try booting the Windows 7 DVD and use the repair option. Did I mention to make a backup first? I can't say 100% every time the repair option will work and you're at the recovery command line, cross that bridge when you get there. (you made a backup, right?)
When you're done, when each system boots it should be drive C:
You can use Disk Management (found in Administrative Tools, or right clicking 'My Computer' and click 'Manage') to choose what partitions appear and what drive letter they appear as.
There are also boot managers like Plop if you want to keep the installations totally separate and not dependent on each other which is what they'd theoretically be if you just installed XP and didn't tinker with the Windows 7 boot manager. Plop would switch out which partition is active based on boot entries you create. It installs / fits in the bootloader with no other requirements so is safe to try out. Personally, for just XP and Win7, the Windows 7 Boot Manager works great so there's no need for it in this situation.
For slipstreaming the Intel AHCI/RAID (RST) drivers, you can use nLite to make a custom XP install disk. https://www.nliteos.com/nlite.html
It's very simple to do, while there, you have the option to spend a bit of time configuring settings in advance, removing stuff you're certain you'll never use, disabling/removing services, drivers and components to have a very fast booting system with low RAM utilisation.
Intel RST drivers v11.2.0.1006 are the "best", this guy said so 😉 https://www.win-raid.com/t25f23-Which-are-the … ID-drivers.html
(seriously though, they are)
With your system, the XP boot screen should be a brief flash on the screen, the bar get's maybe 10% across - on a slimmed down fresh install with carefully chosen drivers and services you didn't remove but aren't using, set to manual or disabled as appropriate.
MAKE A BACKUP
Screw period correct; I wanted a faster system back then. I choose no dropped frames, super fast loading, fully compatible and quiet operation.