Try restarting and hold the Shift key down immediately. Keep it held down through the whole startup process. It will take much longer to startup, but that's normal. That will startup the computer in Safe Boot mode and may help. Restart again (hoping you can get to the Apple menu and tell it to restart normally) and see if a normal startup (no Shift key) happens this time.
The Safe Boot clears out cache files that can get corrupt and cause problems.
If you do have luck getting to a useful desktop in Safe Boot, but still no love with a regular startup, I recommend re-applying the 10.6.7 combo updater.
Do you have access to your 10.6 installer DVD? You can reinstall 10.6, I'm betting that will help.
Side note: Disk Warrior is a great utility to have and I recommend getting it just to have for emergencies like this.
Do you have another Mac nearby? If you start this bad computer up with the "T" key held down it will boot the computer into "Target Disk Mode". Then you can connect it to the other Mac with a firewire cable and run Disk Utility (Aplications-->Utilities) on that drive (it will show as another drive icon on the good Mac desktop). You could also re-apply the 10.6.7 combo updater, found here: