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.
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.