|So it never makes it to the login screen?|
Can you get in using Safe Mode? Restart computer and press F8 to select Safe Mode. If you can get in using safe mode, you will be able to run some diagnostics to determine what is causing the problem. If you can pin the problem down to a certain date, then you can try System Restore to roll back to a couple days before the problem started and see if that solves your problem.