Help! Virus doing my head in!

I constantly get IE opening up to weird adds, sometimes not even a correct address, even though i’m using firefox at the time.

I had AVG Antivirus on the pc but this failed to do anything when I ran a scan so I managed to install a beta F-Secure(Im using Vista by the way) but after doing a scan it still happens.

I’ve also tried using Advanced Spyware Remover which removed some stuff but not this fault.

When I had AVG installed it come up saying that there was an infected file called something like"Trojan Horse Back Door 6." but couldn’t remove it.

Any suggestions?