Hi...
In line with Igor's question, try downloading a new copy of avast and uninstalling and reinstalling from scratch. I've seen this help in other threads, it might with this problem.
Is any of your other security programs having this issue in any way?
Also, we can look at the problem from a possible infection angle. Using Internet Explorer, go here...
http://www.ewido.net/en/onlinescan/You may need to accept an ActiveX control for it to work. Please delete anything it finds, particularly anything marked with a red circle left of the entry. Please post back here with the results and a log, if any entries besides cookies are found.
Best Regards...