Nothing beats a fully fresh windows install, though 
-f0dder
I thought so too... but recently had to switch HD and chose to re-install fom scratch and move it up to SP3. I even googled around on the best order to install programs (IE, Office, SP2, SP3, etc). My portable was 5 years old and working just fine.
My installation is just 2 weeks old now and there are a lot of things which are just not working as before. I suspect that SP3 is to blame for these, but they are so irritating that I'm considering rebuilding again (this time stopping at SP2).
Examples are:
- some high priority process seems to run every 3 sec. or so, preventing mouse move and making sound files choppy
- Only 1 process can send POP requests
- wheel scrolling sometimes has 1 sec delay
- Seems to be more HD activity
- MSAccess sometimes jams for 5 seconds while doing nothing special, unresponsive (I've dev. app with MSAccess for 15 years, so I know it well and have never seen this before)
- I installed Live Mesh and later on removed it and now I have a 2nd monitor named: "Live Mesh Remote Desktop Curtain Driver"

and apparently no way of removing it.
And the list goes on...