Keep it simple and however you do it install XP. You can't install it over Vista. You have to wipe the HD clean. Reformat it. XP Home has been bullet proof for me over several years with a handfull of freeze ups attributed to a zillion things open at once multi-tasking.

But you MUST run clean up utilities no matter what OS your run. Clean the browser cache or your machine will drag ass. Run Disc cleanup and defrag weekly. I use a couple freeware utilities that can do a bit more also.

DO NOT discount the value of having a 2nd HD on your PC. I have for many years. You can easily back things up or out lesser used programs there to keep C drive lean and mean. It is dead simple on a new machine to put the old HD in as drive D slave and simply drag files to new locations instead of endless copying to move them. Also you can install a different OS on the other drive if you have any issues with certain programs not running on a later OS. At boot up you can usually key into the boot menu and choose the which HD you want to boot with. Wanna run Vista and Windows 98? Knock yourself out.