You might have a virus that your anti-virus isn't picking up, no anti-virus program is perfect.
I don't know much about the detection rate with the anti-virus program you have but you can try Kaspersky's Online Scanner
, I know that they are very well trusted. NOTE that this only a scanner and does not remove any threats it finds.
You can also post in the HijackThis Logs and Virus/Trojan/Spyware/Malware Removal
and have a someone there assist you. Before you do so though you would need to download HiJackThis
and it would be best to create a folder in your C: drive labeled HJT so that you know where to access the program's logs and backups.