Microsoft Security Essentials is what I used to use before I got Trend Micro Titanium Max Security. It's free and has all the features anti virus and malware wise that premium anti virus's have infact there was an article on the web about most antivirus's missing this virus anyway then all the anti virus's got updated and bye bye virus they all flagged it. However MSE was one of the first without an update to flag the issue so I recommend it. It gets rid of Root kits virus's malware and anything bad. As for stopping this from happening if it is a hacker I recommend encrypting your HDD and also password protecting your folders and/or OS via Truecrypt. It's a free encryption program and I will post a tutorial later on how to use it