Download.com has a couple hundred thousand free downloads. AVG free 8.5 is a good choice if you can find it. They want you to buy the other program but keep going to AVG free. I have used it for years and it is better than norton, and the others I have used.
AVG, Norton, McAfee, etc. are not powerful enough to totally and completely remove 99% of malicious rootkits, though. Not to mention that the last two are memory hogs.