Most likely a damaged or altered registry entry. Clicks start and type
'regedit' and hit <enter>. Expand the keys at left to reach:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
Click on this key, then locate the shell string in the right pane. Double
click it, replace the contents with just: explorer.exe
Click ok, then close the registry editor and reboot.