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.