Do you have a second computer that's on the same network? If so, boot the locked computer and wait until it's at the logon screen. Once there, on the second computer, go to start --> Run, then enter the command: <b>\\<computername>\c$</b>
Where <b><computername></b> is the name of your computer. i.e. in my case, it's: \\SNAIL\C$
It'll ask you for a username and password, enter the credentials for an <b>Administrator</b> account. Once this is done, an explorer window will open up with the C: drive on the locked computer. This will let you copy any data over to the other machine before you erase the hard drive.
Once that's done, and you have the new version of XP, just put the CD in the drive-tray and reboot the machine. If needed, go into the BIOS to turn on "Boot from CD" (turn it on, and put the CD-ROM option ahead of the Hard Drive option in the boot list). Once you're booted from the CD-ROM, follow the instructions on screen. Everntually you'll be presented with the option of deleting the existing partition and creating a new partition. Just delete the partition that Windows is installed on, Create a new partition once the delete is done in the same place, and you'll be good to go.