If your system is actually alive, you can press ctrl+alt+f1 to go to a console, login as root, do a top and kill the guilty process.
However i think if ctrl-alt-bks doesn't work, you have some low level problem, so there's actually nothing to do
I had a similar problem: from time to time, the sound card didn't disconnect from pci bus, so the system froze, not being able even to access ram! I solved it (after several months) when creative released a firmware upgrade.