[speck]

Take out the battery for a minute or so, then re-insert.

This is going to sound weird - but my Dell Latitude laptop does the same thing. Every once and a while the entire screen will scramble itself - it looks like Yars Revenge, for anyone old enough to remember the Atari 2600. Rebooting doesn't help and sometimes just freezes without even trying to load windows (XP pro). What works is taking out the battery and disconnecting from power for a few minutes.