I've laptop computers with XP and with Vista. I've not noticed any "slowing" on either computer. My latest runs Vista; it started with 140 GB and is now standing at 68 free. It still runs as fast as when I got it (December '07).

Defrag'ing, cache clearing, etc., might help when a 'puter seems to be running too darn slow.

Most of the probs. I've heard about with Vista are related to installing it on older computers... orginally XP or prior platform-compatible.

Vista isn't perfect, yet, it does perform fairly well as long as older software versions will also work - without upgrading. Some software apps. do have patches available, though.