Ubuntu 10.10 and 11.04

Whenever I play a video in just about any player program if I attempt to resize the video screen, or indeed do anything to the program Ubuntu crashes and logs out. Only started a month or so ago on one machine (an old 'Shuttle'). Tried reinstalling Ubuntu again, even with a complete new install onto a blank partition, and it happens with 10.10 and 11.04. Have downloaded new Ubuntu iso images and still the same result. I tried installing Debian Squeeze which I thought was very similar to Ubuntu- works just fine with no problems, though the small differences between the 2 systems are interesting...
Any ideas as to what's happening?