I have a laptop that displays the login screen and then will sit at a blank screen. C+A+D still works, so I checked the running processes. I noticed that explorer was not running, so I attempted to start it manually, which resulted in a "you don't have permission" error. I pulled the HDD and checked it using an external enclosure and it turns out all the permissions for explorer.exe (just this file) have been removed by some virus.
Has anybody seen this before, I suspect its some form of Malware, I had seen something like this once before at work, however it allowed windows to load and then proceeded to show pop-ups all day. It would change the permissions of any executable file that ran. If you ran Word it would close it down after a few seconds and change the permissions, same for virus scanners.