Scott Hanselman has some great tips for cleaning up disk space under Windows Vista. The best tip is using vsp1cln.exe to remove the sp1 install files.

vsp1cln.exe - After you install Vista SP1, it leaves around the original files so you can uninstall the Service Pack if you want. After a few months with the Service Pack, I've decided for myself that it's a good thing and decided I don't need the option.
Open up an administrative command prompt. That means, click the Start Menu, type cmd.exe, then right-click on it and click "Run as Administrator." Alternatively, you can press Ctrl-Shift-Enter to run something as Administrator.
Next, type "vsp1cln" at the command prompt. If you select yes, you'll get back around 2 to 3 gigs. The only thing, again is that you can't uninstall SP1.