Same as me then, ok I managed to log in without having to run system restore by doing this;
1. When your comp is booting press F8 until you get the advanced boot up menu
2. One of the options should be "Enable low resolution video (640x480)" - choose that
3. You should now be able to see the login screen, login and change the resolution to one you know that works.
I've got no idea why it does it either, because its done it to me about 8 times now.
It's forcing me to play in windowed mode, which would be fine if the problem wasn't there with Fullscreen Windowed mode.