Trojan horse PSW Agent AMUT

AVG just ran its normal Friday Scan and detected and healed this Trojan horse. BUT since everything says it comes usually from infected Porn or Video sites, neither of which have ever been visited from this machine, I was wondering where else it could have come from. Also wondering why neither Windows Defender which is permanently ON and supposed to check everything, Spybot, ditto and Scotty did not spot it and warn us?

 

Computer viruses dont come only from Porn sites. Many of them are 'transmitted' via emails attachments, files sharing (music, movies) and the obvious ones like porn/betting/etc.
When you go to an unfamiliar website check that the web address on the status bar matches the address you are visiting and that if you are visiting secure sites they have an s at the end of http = https and a padlock on the status bar.
Any antivirus on the market will protect you 100% is just a case of being up to date with the latest version and hope you dont get targeted with a particularity bad one that is difficult to remove.

 

We have not downloaded anything for weeks except Updates to Windows and Firefox. We have a set of Add ons which are supposed to flag these things. We have actually had no Emails with attachments either and the only sites visited are ones we have been going to for a long time.