Yeah, I didnt experience big slowdowns unless i played with the OS trying all kind of software.

If you dont do that and use regularly a defragmenter (diskeeper is the best) and a cleaner (CC cleaner and Registry mechanic are my favourite), you could go years without a reinstall.

My OS history:
-laptop: winME (crap), win2000 (stable and not a resource hog)
-music rig: XP Pro (didnt last long), Server 2003 (rock solid), Vista x86 RC1, Vista x86 ultimate retail
-gaming rig: Server 2003 32bit SP2 & Vista x64 ultimate (dual boot)