I've had life long installations on all of my computers. I keep them maintained and "optimized" enough where they stay brand new in speed compared to the time they were bought, no hardware upgrades what so ever. Heck, one of my slowest computers still runs very fast, even though it's over 8 years old now lol, and it's last upgrade was an Ethenet card as well so that it could get put on the network. That was the only thing I ever upgraded on it, besides adding 64MB more of RAM to it (which it needed, as 128MB wasn't enough lol). If anyone wants to know the specs of that one comp, PM me and I'll send them to you.
Regarding Viruses/Worms, I have a system here, which is operated by a Linux box which I've found helps keep that stuff from getting to the computer, from blocking ads to a whole mess of sites that I've gotten on a blacklist that are known to mess stuff up, to preventing Peer-to-peer programs from getting any connection at all. Otherwise, AVG Free is on most of my computers (the Windows 2000 machines and the gaming Vista machine), Norton Internet Security 2007 (free from ISP) is on my only Windows XP machine. So it's pretty secure here, so there's nothing to worry about. Also I have gotten, before I got DSL a little over two years ago (grr... all of my viruses came over friggin dial-up!!!!), I got a few unremovable viruses, that I managed to remove by hand. Don't ask me how long that took though, it wasn't a matter of deleting and overwriting files.
Also, about fragmentation, NTFS formatted drives help keep fragmentation down, so there's a tip for you guys.