It is possible that if one avoided all sources of spyware, ad-ware and virus infection that infection would be at minimal levels. Although I don’t recommend using only this method as a form of fighting spyware, it can be a potentially effective mean. Used in conjunction with a virus scanning program (I recommend Avast), abstaining from visiting these web sites or using these programs will help your PC to fight off infections, infestations and slower speeds due to unneeded running processes.
Another good spot to accumulate spyware, especially trojans, is peer to peer file sharing software. Such programs like Bearshare, ARES, Kazaa, etc. have been known to be breeding grounds for trojan horses and keyloggers for sometime. I recommend if you decide to use these programs, you use the “lite” version of the program. Lite versions of said programs have been scaled down in memory usage and the like. They will still operate just as well as the full program, but your computer will like you better for not subjecting it to unknown dangers.
And alas, the final spot of interest would be chain mail letters with attachments. These annoying buggers are usually sent to you by friends with too much time on their hands. Some people become addicted to chain mail letters; they send out every single one they receive. These letters sometimes can have viruses in their attachments, but because the average computer user does not realize it, he or she is spreading a virus. ALSO NOTE: Chain mail letters have been known to connect pedophiles with their prey. This is because the person who receives the letter can trace it back to every single person who has read it. Notice a chain mail letter has tens, maybe hundreds of e-mail addresses upon it, and sometimes these belong to young people (as young people make up a large portion of Internet users). Use a virus scanning program that checks your incoming mail (Avast will do this), and use something like NetNanny to make sure your kids have no strange men or women talking to them on MSN messenger.