Pete in ME - and any one else that's of his mindset; please practice safe computing and be a good netizen - get an AV product and keep it up to date!
I'll also hazard a guess that you haven't done any Microsoft (assuming that's your operating system) updates since you got your computer - most people don't. By not doing the updates and not having anti-virus software on your computer (unless you're running Linux or MacOS - then it's a bit better) you're opening yourself up to mal-ware of all sorts. The AV software accounts for minimal performance delay. What will account for MAJOR performance hits are all the spyware, adware, bots, trojans, worms and viruses (or is that virii?) that will make themselves at home on your computer when you visit some of those sites you do - and they don't have to be the really bad ones and you don't have to open e-mail attachments to get them, either.
I know of what I speak - I fix computers all the time that haven't been updated and haven't used up-to-date AV products. Of course, it keeps me in really nice $$$ so I can enjoy my Stovebolt hobby!
Then there's the [censored] (or 419) letter (here's some information on it -
http://www.secretservice.gov/alert419.shtml) which started this thread. And here's how to report it -
http://www.crimes-of-persuasion.com/[censored]/reporting.htm. And yes, the Secret Service does keep track of these. By the way, they're not all from or regarding [censored] either as I seen some from Sri Lanka, Indonesia, The Congo and Brazil, among others.
Let me urge everyone to get and maintain a quality anti-virus product on their computer and keep your operating system up-to-date with the vendor patches. We'll all be the better for it.
If anyone has a question regarding this topic, please feel free to private message me or ask on the forum and I'll answer as I can.
Sorry for the long reply, but it's something I feel very strongly about.
Jim