Even once you have cleaned your PC and installed a good AV package and firewall you will still get some forms of spyware merely by browsing the web. However, setting up good security will reduce spyware greatly. There are some good anti-spyware packages out there and some poor ones. One free one I tried actually cleaned up the spyware on my PC and then secretly installed spyware of its own. I only discovered it when I bought a spyware package. The advantage to buying an anti-spyware package rather than using a free one is that usually the spyware signatures will be kept more up to date.