What may have happened is an error and a kernel panic, filling up the logfiles.
What I recommend is first to find a way to backup your data without booting the hard drive directly if it isn't already backed up, and then get Alsoft Disk Warrior to repair the hard drive. You'll probably want an external drive from http://www.macsales.com/
And you'll want to go into Hard drive -> Library -> Logs, System -> Library -> Logs, and Users -> yourname -> Library -> Logs to remove whatever you find there.
If after booting it fills up again, start by removing all startup items in your System Preferences for your user, and rebooting in safe mode with the shift key held down. If the hard drive is over 85% full after removing the logfiles, you may want to look at what were the last applications you installed.
Peer2peer software if installed can be as bad as a virus, because it enables others to access your computer. Insecure wireless networks are just as bad, if they are WEP or no encryption and someone is within 1500 feet of your computer.
If no such software is installed, your RAM probably has failed. Run the hardware test that came with your computer, by booting with the 'D' key.
If nothing shows up there, it can still be the RAM. Especially if you've seen any 4 language screens telling you to restart.
Here's more on kernel panics:
http://www.macmaps.com/kernelpanic.html*