[TwoSheds]

XP has the "windows rot" problem. After it has been in use for a couple of months, it slows down significantly and especially booting it takes forever. Some people claim that they can avoid this but I have never seen any hard evidence :)

[Maro]

There's a free tool called Sysinternals Autoruns (autoruns.exe) which lets you remove all kinds of startup/resident crap. I use it to clean up every couple of months, and as a result my XP is as fast as after a fresh install.

[rbanffy]

There is pagedefrag, also from the sysinternals folks. That said, Linux doesn't have a similar Linux rot problem. I have some installs that have been going since 2002 or so, with successive "apt-get dist-upgrade"s crossing releases without much significant trouble except a ssh key regen.

[norswap]

I manage to do pretty well. A good uninstaller (which performs deep scans), a defragmenter (MyDefrag is best) and CCleaner will do the trick.

But it's true that a little reinstall once every few years can't give a slight speedup.

Nowadays I tend toward a nomad setup (using portable apps) anyway, so it's even easier.