My personal pet peeve is that Windoze seems to degrade with time, no matter HOW much security patches, anti-spyware etc. etc. you do or run - it just doesn't make any difference. Maybe some server running in the closet doesn't suffer from this malady but a basic Windoze machine is going to get slower and slower and slower until it finally grinds to a halt. I don't know if this is registry bloat, or swapfile fragmentation, or the heartbreak of psoriasis but it just seems to be a universal law of the universe of computing. If Windoze doesn't fix this issue they are digging their own grave eventually.
I personally wonder how many new computers have been purchased because the old one "gave out" when, in fact, the hardware as perfectly fine but it was really Windoze that "gave out"?
It's called "WinRot."
Keep your registry and startup clean (I recommend EasyCleaner by ToniArts), and defragment regularly (the built-in Windows tool is fine for this). In conjunction with keeping your system virus- and spyware-free, doing the above should keep your system running as fast as it was when new. Faster, actually, since a big part of keeping the registry and startup clean is getting rid of a lot of the crap that every manufacturer seems to want to saddle new machines with.