after checking google most of the replies refer to malware.
often the change is made to the registry so that should be your next stop.
if it has been changed then you will need better scanning. with dedicated antirootkit apps.
aviras is ok but its not the best for rootkits. Sophos Anti-Rootkit. is a very good 1 as are gmer and rootrepeal...
i also recommend you run hijack this and post your results on there forum.
do not use it try to fix it by yourself as it can easily wreck your system if you remove something you shouldn't. there guys will look at the logs and give you either advice on what to delete or recommend another antimalware app that will hopefully get rid of the problem... good luck.http://forums.techguy.org/
have a dedicated section for posting hijack this logs.