This problem is due to a virus. What you need to do is delete the explorer key in the registry. CTRL + ALT + DEL and bring up task manager, file>new task (run) and regedit - HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\Explorer.exe When you try to delete this it will throw up an error telling you you don't have permissions. Drill into the properties and take ownership (assuming you're an local admin), then reapply permissions by inheritance. You should now be in a position to delete this key. Log off and it should now log in fine.
