Is this a workstation/member server or a former DC? If the former, try using the original workgroup (usually the name of the computer at installation) and the administrator of that group as the logon. If the latter, just try the original installation password with whatever the workgroup name is now. Sometimes the objects get skewed and you lose the ability to logon (I have had this happen a couple of times with NT4 and once with WinXP Pro), so you have to bypass and reset the password. I have used this utility here:
 
http://home.eunet.no/~pnordahl/ntpasswd/
 
that was pointed out in a post from a while ago by mlazovjp. If you use it, don't disable SYSKEY if you don't have to (you shouldn't), and make sure to read the directions FIRST. It works pretty well, but I did have some "side effects" after usage. Basically, I would backup all of my important data and make sure that anything relying on certificates and keys are handled appropriately in case it does have issues later on.