Antivir Personal Edition: www.free-av.com
it is light on resources even on my 500MHZ system, really fast at detecting viruses, it has heuristics which detects a lot of viruses that are not in its definitions
AVG is the shit.. seriously, they're a badass av.. I use them with house call (for trojans) and both of those combined work great.. I've used both of those for nearly 2 years for any clients computer that needed it.
Here's a link for housecall, it's an online virus scan.. just use it if you think you're infected or maybe once a week anyways..