Intercheck, which is part of Sophos, is known for this. Try turning sophos off and restart.
If that doesn't fix it, take a look at this: Mac OS X: How to troubleshoot a software issue
1. make a new test user and see if the problem persists.
2. repair your hard drive and permissions
2.5 try a safe boot again
3. do the cache folder thing recommended in the article above.
4. if that doesn't work: reinstall OSX