So far the known culprit list includes:
Norton Antivirus, F-Secure Antivirus, and Spampal
(edit: plus Internet Explorer 8 beta 2 IIRC)
If you are using one of these, please disable it and make sure it is not running in resident memory and attempt to reinstall and it may resolve your issue.
If you find out it was another program, please send me a PM and let me know I'm trying to keep a list of known offenders (and while I'm going to keep checking in on this thread, a PM is faster