Comment 1 for bug 64621

Which screen saver is the machine configured to use? Some of them are VERY processor intensive, and with a slow CPU and no GPU the system will be brough to its knees. I think if you waited, maybe a long time, it would come back. And it is not really a system crash if you can kill X windows and get back to the login.
Try setting it to just "blank screen" and see if it does not make the machine unusable.
Obviously, there should be some way for a screensaver to "know" when it is not able to run on a machine.