I forgotten my password a few times, and it has been changed when i dont want it to be i.e. virus's. a few methods i've used are 1) At the welcome screen, press control alt delete a couple times to change the logon window, enter 'Administrator' for the username and nothng for the password. if that dont work. 2) Press F8 at the boot screen and boot your computer up in safe mode. This automatically gets you in as administrator and you can do whatever needs to be done. 3) my last option, this is a security hole in windows xp's software. You need to have a OS cd for your computer. boot up on the disk, at the welcome to setup screen, press enter to setup windows now. next, hit F8 to agree, then at the setup screen press r to repair. next, after the reboot wait for the installation label to say 'Installing Devices', when you see that, press Shitf+F10. at the command prompt type in NUSRMGR.CPL and press Enter. You now have access to the user accounts. When finished, exit the command prompt and finish with the repair. After all this work, i suggest you make a password reset disk.
*NOTE:Make sure you have your product key*