Try shutting down your computer, and when the OEM's logo comes up wait until the screen goes blank, then hold down the F8 key and select Safe Mode, when the login screen appears, try logging in with your normal account, if you have no luck, use the built in Administrator account and try to recover any lost data. After you have backed everything up, try a system restore and if you have no luck then Boot from the Windows 7 DVD and select repair my computer during install, this utility won't erase anything, select repair and if it doesn't work after all of this, re-install windows 7.
Unfortuantely i had a similar issue to this and after all my research and talk with professionals who know windows 7 it was determined that the only fix for this issue was to reinstall the operating system. Now remember that was for the Windows 7 RTM version there may be a fix since that was released i cant be sure.
Just in case, if you were going down this road, I would suggest using the Repair startup first. If that doesn't work, and it may take several attempts, then do a system restore. Hopefully one of these will correct the problem so you don;t have to go any further.
I had a similar problem with windows 10, receiving a "unable to locate user profile" error, a little digging discovered my startup options had
changed for some reason, Using the computer management administrative tools dialogue I reverted to the "normal startup" from the
"selective startup" For windows 10 the answer was here: Start Menu>WIndows Administrative tools>System Configuration>General