How does this nasty get past one's existing anti virus software?
Because most AV software is very good at slowing your system down but often miss some virii. No AV software will catch everything despite the writer's extravagant claims.
AVG is free but clunky in later editions. Avast! is free and as effective as any without a large overhead.
Zone Alarm is almost a complete waste of time especially if you are behind a router.
Malwarebytes is a good program but all the people here extolling its virtues are omitting the fact that it will not necessarily rid you of ALL malicious software. Hence my link to Majorgeeks and the step by step procedures you need to go through to be certain to find everything causing problems or lurking unseen.