I have seen on some old Dimension desktops the boot partition is actually not the C: drive contining Windows, plus they had that crappy diagnostic / recovery partition at the end of the drive. If you’re doing a full nuke and reinstall this won’t affect you though. You could even do it on a brand new hard drive, if you wanted to save the Vista install for some reason. The only problem you’re likely to encounter is if some of the hardware is from the Vista era, XP device drivers may not exist.
Good idea. This is what I did with my ancient Dell laptop. I bought a new disk from Frys, a new drive caddy from ebay, and installed Linux on it. I still have the old Win2K disk laying around somewhere.