If you cannot afford to pay for anything, here are some simple, excellent programs that will protect your system and best of all, they're FREE!
Anti-Virus: AVG Free
Spyware: Spybot Search & Destroy, SpywareBlaster, AdAware 2007
I should but do not use a software firewall. ZoneAlarm was a good one. If it was just me using it, I would install it, but with other novice users using the same computer, for me it was just a hassle trying to set up the right permissions for it to work without it being too Big Brother-ish and the user coming to me because they were too afraid they did something wrong.
If you're planning on downloading torrents, I would suggest to never download and use programs that are information senstive (such as Money, Quicken) and never download pirated virus/adware/spybot program. You're better off using free software because you never know what kind of infection was planted inside of these programs.
Happy porn-downloading!