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I use a hardware solution for my firewall. Just about any D-Link router or switch will do the trick. I hate software solutions, they're buggy, eat system resources and can be vulnerable.
I don't use antvirus, spybot or adaware. A good firewall and smart computer useage are key to a clean system. Of course I'm meticulous about my computer(s) and my methods come from years of experience and is not recommended for casual users.
Ad-Aware is good for picking up little bit's and peices here and there, use it often if you have it. Spybot is a nice tool if you know what your doing and use backups in case of a mistake. It's powerfull and can take down your system if your not carefull.
Antivirus is a no brainer, but if your a system performace freak like me and not using an antivirus is not to your likeing, I highly recommend to family, my users and support clients to use AVG. It's free and very low on system resources. The paid version is well worth the investment, however it's email support is a pain to set up depending on your email client. For the less computer savvy people I recomend Norton. Simply because of it's out of the box solutions, although I prefer to avoid the system resource hog on low end machines. IT drastically slows down boot up times, system access times and takes a far chunk of memory through it's services.
Carefull computing is alway's key and constant awareness of whats being installed on your system will go a long way at keeping your system clean. I currently maintain a small network of computers at my fathers place, they're old p333's and only have 128mb RAM, yet I am able to run XP Pro in a lean environment with a little protection from AVG and behind a hardware firewall. Boot up takes less then 100mb of ram, they're clean of virus's and bad active-x objects and other browser hijacks and computer nasties. They're not just a family computers, it's a computer network for a daycare of some 12 kids who surf and play games daily.
Just a case in point that you can run a lean system if your diligent enough.
Happy Computing!