Boot into safe mode. Press F8
before the Windows XP loading splash screen.
When the welcome screen appears, you should see the account Administrator
at the top. This is the system administrator account, above all other accounts. Normally you won't see this.
Click to log into this account, entering a password if you're prompted for one (it should be the password specified upon the installation of Windows XP).
If you're not presented with a welcome screen, but a login box instead (because fast user switching is disabled), then just type Administrator
as the user account name, and the corresponding password.
If you manage to gain access to the system admin account, then go to User Accounts in Control Panel and change the password for your user account.
If you can't access the system administrator account, then there is a tool you can use to change Windows XP passwords:
The top section is a step by step guide. At the bottom, there's a section named Downloads
. Download either the Bootdisc Image (CD Image) or the Bootdisk Image (Floppy Disk Image) to create a bootable CD / Floppy Disk...
...then, place the chosen medium into the drive, reboot the PC. Check in your BIOS that either the CD-ROM drive or Floppy Disk drive is set as the primary boot device (depending on which medium you've chosen), and then boot into it.
Follow the on screen instructions. It may seem confusing, but if you print out the step by step instructions, you should get through it easily.