hey mate,
have you tried booting the computer into safe mode?
when you boot the computer up in normal mode! you will see the user list appear and you normally select the correct one and enter your password(if it is set up)
when you turn the computer on! press F8 repeatedly after the initial start up, you should then get a menu that displays, you'l have choices like, safe mode, safe mode with networking, last known good configuration, etc...
select safe mode and when the user screen displays, there will be an extra user called ADMINISTRATOR. try this account to reset everything. if the account needs a password, i have a nifty little disc that resets windows xp passwords, there is not always a need to reinstall! if you need a copy of the disc then i'll have to upload the .iso somewhere and you'll have to find somewhere it burn it to disc! the password revovery software is free on the net but i cant remember where i got it from!