It's a good idea to reinstall your operating system and drivers once a year or so just to keep things fresh. Also, it's not until after a good formatting of your hard disk that you realize just how much of the shit you had on there was useless.
All of my important do
ents are on flash drive and/or phone and/or in the so-called "cloud." Keeping them just on the computer is begging for trouble.
As for Win 7, I agree that it's great. I'm running 32-bit Ultimate and if you were to sit at my desk and use it, you'd never know it's running on a 6 year old computer.
EDIT: Also thought to add another tidbit. For the love of god, don't just leave your porn and "intimate" photos or anything else even the least bit incriminating or embarrassing on your computer. When I work on computers, I will never touch a file I don't have to touch and respect privacy. That said, there are cases when that data has to be handled or is simply stumbled upon. Also, having worked in a few computer shops in the past, I've seen many paid techs out there will snoop for entertainment. Basically, if feasible, don't have anything on your hard drive you wouldn't want another person seeing. You never know when you might find yourself in a situation that requires letting a professional nerd wrench around on your box.