I would try a combination of using fullscreen/window modes. I've personally had my client crash if I've tried to do any special alt tabbing while critical loads take place but that does not sound like your problem. Who knows, possibly making sure your video drivers are updated as well could be a solution. It's strange that it goes from working to not working. Can you think of any other software/hardware that was added or removed from your system?
Obviously a last resort would be to start with a fresh client but thats a PITA. Unless you have all the isos and then the install is fairly quick.
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