Thing is, you can't simply downgrade an OS. It's not like install SP2 for XP, no uninstall option...
Best thing to do, imo, is to buy a 2nd HDD, master it, slave your old one, install XP SP2 on the new HDD, then copy everything (pix, flix, docs, etc.) from the old, onto the new drive.
Then use the old HDD as a backup HDD for those files. Could buy a USB external enclosure to have it as a true backup. Because if you have two internal HDDs, if they crash by power failure, and the head drop down on one disc, more than likely going to do it with both, so then both are worthly.
BTW, I'll take you upgrade to Vista... Is it Ultimate? One of my techy friends failed to give me a OEM copy of Vista Ultimate...