Try to boot on Mac OS X CD1 (Press C during boot), then launch Disk Utility form the Installer menu, and try to repair the disk. Then repair permissions.
If it's still not better, you can try to delete preference files in Library/Preferences of your user for applications where you have problems.
If all this doesn't help, just create a new user... (You will need to create a new administrator account from your current account, then login with the new account, and delete the old one).