Trojans, Malicious Code, You Name it. We've found the two best pieces of software that can provide a lot of protection without putting a dent in your wallet. These don't mix with each other - so pick no more than one.
AVG Anti-Virus
Pros: Free, Fast & Light, Basic Coverage Cons: Less Functionality
Avast Anti-Virus
Pros: Free, Fast, Comprehensive Cons: More Memory, Slightly Oversensitive