When disk space is running out on a system disk, may it be on a server or a client, there are certain things to clean out. One of them being the %SYSTEMDRIVE%\Windows\Installer folder. You cannot under any circumstances delete files from this folder manually as this not only may but most likely will break software that is installed using MSI files, or Windows Installer files.
The %SYSTEMDRIVE%\Windows\Installer folder is a cache for installation files and patches (MSP files) and removing those will cause you to not being able to repair or uninstall applications, and in some cases not removing patches or applying new patches to software. In the event when you actually did delete this cache you can rebuild the files you need manually by extracting the files from original installation media, from patch packages etc but this is a time consuming and not that easy task to accomplish.
But let me get to the point. If you do want to free disk space you can clean out the %SYSTEMDRIVE%\Windows\Installer folder by downloading Windows Installer Cleanup Utility (NOTE: This tool has been retired and is no longer available from Microsoft) and then running the command
When running this, the installer and patch packages are enumerated and unreferenced packages are considered to be safe to delete and are thereby also deleted. Depending on the age of the system and the number of applications installed, this action can free a significant amount of disk space.