For Vista there are several things you can do after installing SP2. Note that the following steps will make SP2 permanent on your system (no uninstall possible).
1) Be sure all updates are installed.
2) To remove SP1 junk files:
open a command prompt and execute the command "vsp1cln.exe" and answer with 'Y'
3) To remove SP2 junk files:
In the same command prompt execute the command "compcln.exe" and answer with 'Y'
Let the Microsoft tools do it's things.
4) The following is at your own risk (be sure to make a backup). Go to the "\Windows\SoftwareDistribution\Download" folder. These are the downloaded update files. Delete all files and folders in there.
5) Press windowskey-R and type 'cleanmgr /sageset:50' and select the junk you want to get removed. This is not the actual deletion and '50' is just a number. To do the actual clean-up execute the command 'cleanmgr /sagerun:50'.