I can’t logon to local admin/system.

I can’t logon to local admin [log on to :mycomputer (this computer)].i’m using windows 2000 server NT.And my problem is i had try to logon to local admin several times with correct user and password but its always returning me to logon screen again,also its was happened with same thing while i had try logon at safe mode.pls help me on this.your effort i’m very apprecited.thanks very much..:)