In the event this happens, you can often boot into safe mode and enter the account. Once you enter the account, open the registry. (regedit)
Navigate to the following-
Expand the left Window and you may see a profile with a .bak extension.
Remove the .bak from the SID name. (Take away the .bak only)
Reboot. You should be able to login to Windows in normal mode.
Other troubleshooting methods –
Note: You may have to boot to the cmd prompt after hitting F8 in the event you cannot get into safe mode under the corrupt profile.
(It is also good to go ahead and do this regardless)
At the cmd prompt, type –
net user administrator /active:yes
net user administrator new_password
Upon rebooting or logging off, you will now see the Administrator account. Use the password you set to login.