%WINDIR%\SoftwareDistribution\Download - delete whatever is older than 10 days there. Later files may still be needed by ongoing Windows Update
%WINDIR%\Installer\$PatchCache$ - mostly safe to delete.
DO NOT however delete whatever is in the root of %WINDIR%\Installer !!
download msizap utility which comes in Windows Installer SDK - http://msdn.microsoft.com/en-us/library/windows/desktop/aa370834(v=vs.85).aspx - and run it with g option, i.e. msizap g - it will delete orphaned windows installer files. Other options of this utility may be risky - general rule is that messing with windows installer files that are in your windows folder may break your system and you will need to reinstall it.
More info on msizap: http://msdn.microsoft.com/en-us/library/windows/desktop/aa370523(v=vs.85).aspx