and is a VERY GOOD REASON to have a second user account with Admin priviledges.
If you do not have a second Admin account but do have a limited user account choose to run CMD.EXE as ADMINISTRATOR and type the command Net user administrator /active:yes
That will activate the hidden default ADMINISTRATOR account.
If you do not have a second account you can boot to you will have to use the Installation DVD and its Recovery Environment to see if a system restore to a known good date will get you in.
Recently I have this message when trying to log into admin account.Hence I can't download/update software.I'm definitely using the correct password. I tried a few things I saw on the net including trying to enable built in admin account but with no success.I did find that my admin user profile had .bak which i guess isn't right but how this has happened ?????

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic