Alrighty. Well offhand, it sounds like you have something seriously corrupted in your system. What it is exactly is tough to say. There's one thing you can try to avoid reinstalling the OS completely, but be sure you do this exactly.
EDIT: Do this first actually... we should make sure the disk drive isn't going bad: Boot from the DVD, exit the OS installer, and run Disk Utility from the DVD. Have it verify the disk (and repair permissions). Directions
here.
If the disk checks out OK but the permissions repair doesn't work, then read on...
Make a copy of /Library... name it Library-Old, or something like that. /Library is the folder you see if you are looking at the contents of your disk drive. Once you are sure you have the right folder and have a backup copy, delete the original /Library. Do NOT delete the Library folder that is found under the System folder, nor the one under your user's home folder. You may have to boot from the install DVD in order to do this. OS X will rebuild the Library from scratch... you'll have a virgin, untainted copy.
With any luck, this will solve your problem. It may break some applications... just reinstall those. Again... be absolutely sure you are deleting the correct Library folder and have the backup just in case. Barring this... I don't an alternative aside from reinstalling. As long as the problem isn't rooted in the System folder somewhere, this should work.