Freeing up space on your hard disks is an ever ongoing battle for some people, for instance if you at some point decided on a too small system partition (“10 Gigs must surely be enough?”).
So in the Windows directory (on Windows XP at least) there is a folder called “Installer” where many install files will be found. It may be tempting to just delete all files here, freeing up many gigs of space, but that could cause problems later, for instance if you want to upgrade some program that needs the old version to be uninstalled first.
So this is where the Windows Installer Cleanup Utility comes into play. Read this excellent blog post on where to get it and how to use it: http://blogs.msdn.com/heaths/archive/2007/01/31/how-to-safely-delete-orphaned-patches.aspx
Essentially after installing the utility, you may run a command that will delete orphaned installation files, that is install files for programs that are no longer on your system.
I managed to free up enough space to install MS VS 2008 Professional… :)