When you promote a member server to domain controller it's local SAM database is locked hence you cannot logon locally and only domain admins are allowed to login to the DC. I'm not sure which
account you're referring to here? Have you forgotten the DSRM password? if yes you can reset it with steps listed in the following link-
How To Reset the Directory Services Restore Mode Administrator Account Password
http://support.microsoft.com/kb/322672And also note that if you plan to demote this domain controller then you will asked to set a new local admin password during demotion wizard so that will solve your problem.
Sachin Gadhave
MCP, MCSA, MCTS
Need to support users over the internet?
click here try our remote control online beta