A lot of the smart viruses and Trojan horses today try really hard and prevent you form going to the main anti-virus and "cleanup" sort of product including microsoft own tools. They actually block your access to those sites.

A couple of weeks ago my wife's laptop got totally infected (she must be browsing porn) and I ended up finding a little known program that did most of the clean up and then followed up with Microsoft tools that are available on their site and successfully cleaned everything with the exception of one item i had to get rid of manually.

Name of the product is Exterminate it. Worked pretty well.

Source

Spoonshadows

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