Just wanted to generally agree.
If you have to ask if your registry needs cleaning, you don't need it. (If you're one of the rare nut-jobs that has actually boogered up your registry to where it "needs" cleaning, you're probably better off reformatting and reinstalling.)
Ignore pop-ups from web sites urging you to download or buy stuff to enhance your computer's performance or security, etc. They should be taken as seriously as a guy selling commodity futures door-to-door.
Defrag every 48 hours? Heck, I usually don't reboot that often. If you NEVER defrag, you are not a bad person. Defrag, disk cleanup, etc. can help some, but it's nowhere near as important as changing your oil or checking tire pressure.
You should, however, have some kind of virus and spyware scanning software, plus some sort of firewall. I have both a software firewall and a router that serves as a rudimentary hardware firewall.
Most other preventive maintenance is BS. I think that now that cars lack chokes, carburators, points, etc., we crave having something to tune up and maintain.
Just make sure to keep air vents clear, plug into a grounded outlet, and be wary of your nephew or other kids "helping" you or otherwise reconfiguring all your stuff.