To the OP typically these types of malware are installed by the user, unintentionally of course but this is by far the most common issue with these kinds of problems.
Anytime you install anything, carefully read what its asking in the install window, always choose custom or advanced install, never just click next next next ok!
Viruses are pretty much an afterthought these days, because malware has been so successful at getting people to agree to them being installed.
This article nicely compiles the common problems most people run into, well worth the read.
http://www.howtogeek.com/howto/2998/...n-my-computer/