I have seen this a lot at work, its not annoying at all!
I manage to fix it by:
Mayhttp://www.sevenforums.com/tutorials/4230-registry-backup-restore.html
1) Click Start
2) Type 'regedit'
3) Press enter
Before going any further, take a backup of the registry -
Registry - Backup and Restore
4) Navigate to - HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList
5) Now delete the line with .bak at the end
6) Restart
Hope this helps.
Regards,
JDobbsy1987
[EDIT]
Brink beat me to it