How do you protect your computer?

[ABV]

I'm going to be loading windows again on my home computer in the next couple of days, because what I have now is FUBAR. I'm not sure what happened with mine, but I think it's a combination of things. First of all, I'm not even sure if I have a legit version of windows on there right now (I just ordered windows XP Home from NewEgg, it should be here soon). I also think that I have some viruses and other crap on there too. So after I format the hard drive, and load my new legit version of Windows, how do I keep it safe? What do you use for virus protection, or a firewall? I thought I knew enough about this to keep things going smooth, but apparently I don't, which is why I'm posting this.

Any help is appreciated.

Thanks
Rickshaw4evr