How do you prevent an application from grinding the whole system to a halt?

[Chanchao]

This annoyed me this morning... Was happy to see that the Dapper
download was like 90% finished and still happily downloading away..
Then had to rotate an image. (2560 x something, 5MP camera shot).
While rotating though, The Gimp managed to bring everything down.
Nothing responded anymore, the clock in the top left remained stationary
at 9:02 am for 20 minutes, I couldn't close Gimp, I couldn't move to a
different workplace, Ctrl-Alt-F1 and so on didn't do anything to give me
a TTY, Ctrl-Alt-Backspace didn't do anything..   So just had to pull the
plug and restart. 

Is there anything you can do to stop applications crashing everything?
Wouldn't the OS take care of such things all by itself in a perfect
world? (i.e. is something wrong with my installation? I do have 512MB
memory installed.)

Cheers,
Chanchao