Just a note if you should decide to buy a new computer with vista many of your old programs, printers, modems, usually are not compatible. It sucks you buy a new computer and find non of your old progams work on vista, like the printer, etc...
I just bought an used 50 dollar gateway with a 1.8 speed and bought another 50 dollars in RAM memory so I would not lose all the programs I paid good money for then ran my xp program unto my harddrive and it working great printer, modem, etc...
If you have at least a 900 speed computer I'd just reload XP onto your existing hard drive and max out your RAM memory? reload all your programs, anti-virus, printers, etc....
P.S. Anything less than 900 might consider buying a faster computer, just my opinion though. If you have the xp software you might beable to buy a faster used computer that you can upgrade personally by downloading your xp software to. Schools sometimes sell off their older computers that probably are far faster than what your now presently using?