If you have some access to the pc that has adminstrative rights, just log into the pc with that credential and change the password locally.
Barring that, you really only have 3 options that I know of:
1) Your best bet is to purchase a program called "ERD Commander 2002" from www.winternals.com. It is a recovery program built for Win2k/XP that lets you do administrative wonder-stuff like change passwords locally as well as some other great stuff. Downside is that it's ~$400 for a single license.
2) l0pht crack. (www.@stake.com). All it does is password recovery. Downside is also ~$300 for a license and can take days if not weeks to bruteforce a password open.
3) Reformat and start over again.
Win2k/XP were built to be protected from doing exactly what you are trying to do otherwise.