How can you change a former admin's password?

There are programs that you can purchase that will attempt to replace the current password on the system with one of your choosing, or with a blank password. These programs are operating system dependent, and most often are directed at the local administrator password, not the domain administrator password in active directory. Two potential sources to check out are:

http://www.lostpassword.com/windows-xp-2000-nt.htm

http://www.iopus.com/password_recovery.htm

I cannot vouch for any specific software, or process or recovery service, these are just a starting point for you. I suggest a web search on the words password recovery, password crack and so forth, to find additional programs. Please do check out the companies -- and their software or services before making a purchase.