Windows XP stores password information in encrypted files called SAM files. A Google search for SAM file editor will return plenty of results, and you'll find several programs that you can burn to a bootable CD and use to edit these files. I've never personally had to use one of these, so I can't recommend one. Using these editors can render your computer unusable if anything goes wrong. The risk involved in doing this makes an absolute last resort. The tiniest error in this will corrupt files and lock you out of Windows at best; it will destroy your Windows install and force a reformat at worst.