Windows Vista is a pretty big install, and then when you add to it the install of Service Pack 1, it becomes even bigger. So, once you have been running Service Pack 1 for a while, you can safely remove the uninstall files for the Service Pack. Doing so should save you around 1Gb of disk space as well! :-)
Simply run the command wsp1cln.exe from an elevate command prompt and then let it do its stuff.
How should I uninstall SP1 after I remove "uninstall files"?
If you have removed the uninstall files, then you'll have to reinstall the operating system if you want to go back to Vista RTM.