I recommend running the Windows Updates installation process before doing this procedure so you don't delete anything that you might have to download again. Windows Updates leaves many files laying around in the C:\WINDOWS\SoftwareDistribution after updating itself. These files can be safely deleted. Simply stop the Automatic Update services and delete all the files in C:\WINDOWS\SoftwareDistribution. However, don't delete the SoftwareDistribution folder. Restart the Automatic Updates service and Windows will rebuild the folder and file structure as needed. If you've never done this, this process could save you several hundred MBs of space.