Someone is going to say "the best protection against viruses is a Mac".
Before you do, it's not funny.
SpyBot
AdAware (As recommended above)
Those are two great (free) spyware programs that might solve some problems. They don't help with viruses, though. I use a payed-for Norton so I'm not familiar with free virus stuff. Either way, I find the two spyware programs are practically a necessesity, and I
hate unnecessary programs cluttering up my computer. If you use the internet, they
will fix things.