The following is my opinion based on my experiences with about 20 man-years of PC computing. Your mileage may, and probably will, vary.
I have never, not once, found a legitimate computer problem on any of the 6 computers in my house, or any for the last 19 years of computing, to be caused by a virus or worm or trojan.
That is not to say that I haven't found these things (expecially trojans) by doing routine scans, but I've never actually been "infected" with one to be precise.
I do NOT run anti-virus software like McAfee, et al (my wifes box has an expired version her company installed) because the programs are more trouble than they are worth and I just don't like them. It doesn't seem to have hurt me much.
Here is what I DO do.
1) Any computer on DSL or cable must, must MUST have a router/gateway to filter out all unsolicited WAN request packets. This alone will get rid of 90% of your problems.
2) Any computer running on dial-up must, must MUST have Zone Alarm or equivalent filter that does essentially the same thing the router/gateway does on DSL.
3) Never, never, don't even think about opening any email attachment that you did not expect to get and know about. Even if it appears to come from a friend, if it looks the least bit fishy, call and verify before you open it. If possible, don't even open an email with a subject/sender that doesn't look like something legit.
4) Don't go downloading and running programs off the net unless you have some reason to believe they are legit.
5) Do a search on your WINDOWS folder for a file named wscript.exe. Rename the file to something else like wscript_.exe. This will prevent certain email executables or scripts in email from executing. This tip came from the lead tech in my wife's help-desk group - I've done it for years and I've seen no negative consequence.
I do occassionally run an online virus scan (Trend House Call) and it occasionally finds a worm or trojan and deletes it.
I do get a reasonable amount of infection from Spyware, which has become almost impossible to avoid completely. I run Ad-Aware periodically to clean up the mess. There are other good programs that do the same.
There are lots of things that can cause a computer to slow to a crawl. It takes a lot of information to diagnose these sorts of problems - for example, does it boot slowly and run slowly all the time? Others here have suggested a bloated registry and that is a definite possibility. Run Task Manager and see how much virtual ram you are using. If it is higher than the physical ram, there is another likely culprit, if you load more into memory than you have memory, you start swapping to disk and that will slow any machine down.
There are numerous other possibilities, including some kind of infection. Google Trend House Call and run their scanner to rule that out.
IMHO, computers are complicated beasts and the OS running them is still not that bulletproof. When things go funky everyone tends to think "virus" when in reality most viruses have better things to do than to just screw with you by slowing your machine down. My wife is a help desk manager and they do get hit about 1-2 times a year, usually from some dummy (putting it politely) opening an infected attachment. Once the thing gets on their local network they often spread pretty quickly and cause a lot of problems. The only nice thing is there is usually nothing subtle about them, they like to make their presence known, that is the whole payoff the author gets /forums/images/graemlins/smile.gif