Linux!!!
(I can say that now I'm officially converted.)
Assuming you wanna stay with MS (which begs the question-why?) try
this site. I think the 5-star-rated ones are equally effective and it comes down to which interface you prefer.
On the various PC's I look after (family & friends), I use Avast!; it's highly rated throughout the industry...and it's free.
I used Comodo's Security Suite (firewall and antivirus) on my PC when I was running Windows.
With any of them, a good antispyware application (with real-time protection) is extra insurance. Again, plenty of decent free ones from the above site.
Also, consider using Firefox with the NoScript extension.
