The Windows Installer Directory, typically located under C:\Windows\Installer.
This directory is needed when applications are installed or updated: it stores the necessary .msi (installer) and .msp (patch) files.
Usually, this directory occupies quite some space on the harddrive and it's tempting to remove files from it - however, its not really safe because some files are needed to uninstall (or update?) applications.
If space becomes a concern, trying to compress the folder might be an option.
TODO
msizap.exe (Windows SDK Components for Windows Installer Developers) is a command line utility that removes either all Windows installer information for a product or all products installed on a computer.