Is this the typical "kernel panic" screen, or some other error? Have you done the typical troubleshooting things (repair disk, repair permissions, swap out non-Apple RAM, etc.)?
Please provide more information (a LOT more). Any recent software additions? How long has this been happening? What's the last change you made to the computer before it exhibited this problem? Any hardware upgrades? Any reason it's not running the latest verison of the OS (10.3.9)?