How to Remove all Remnants of the Windows Vista SP1 Installation
When you install Vista SP1, the installation procedure leaves all of the pre-SP1 files on your system in case you need to uninstall it.
As you can imagine, these files, which are required to allow you to uninstall SP1 and revert back to the original version, can take up a large amount of hard disk space. In fact, the files can take as much as much as 800MB.
However, if you are positive that you want to stick with Vista SP1, you can remove all the leftover files and reclaim approximately 800MB valuable disk space by using the SP1 Files Removal Tool. Fortunately, this tool is automatically installed on your system as a part of the SP1 upgrade procedure.
Before you run the SP1 Files Removal Tool you must be absolutely sure that your system is functioning perfectly with SP1. The reason being once you use the SP1 Files Removal Tool, you won’t be able to uninstall SP1.
If you suspect that your system is encountering compatibility issues with Vista SP1, you may want to hold off on running the SP1 Files Removal Tool, because you may need to remove SP1 as a troubleshooting step. If you do, all of the original files that were replaced by SP1 will be put back into operation and you can explore your troubleshooting options.
Full instructions for removing all remnants of the Windows SP1 installation: Greg Schultz | TechRepublic