If you search the forums, you will find loads of posts
The "best antivirus software" is ... do not connect to the Internet. That will offer 98% protection.
Joking aside, I totally agree with Ady ... KIS, Kaspersky Internet Security is damn good, install it and forget it.
It is, what is commonly called an "all-in-one" solution ... most people think this is not so good an idea, but I'm afraid they are probably victims of Norton or McAfee's poor incarnations.
If you would prefer to use separate programs,
that may not get along, then I can suggest ... KAV
(Kaspersky Anti Virus), NOD32
(an exceptional AV program) and AntiVir, the latter can be had for free.
In conjunction to a "stand alone" AV program, you should install a Firewall ... you will need something a little better than MS's own built-in Firewall,
even if you use a NAT Router, a software firewall is strongly recommended.