|we have a few servers with 10-12GB partitions, which is barely large enough to host the OS with updates and a few apps. But the following are some space-saving techniques. Make sure Folder Options are set to Show Hidden Files.|
1). Go into C:\Windows, and see if there are any folders whose names look like $NtUninstallKB958687$ or some other KB#. These folders are only really necessary if you plan on UNINSTALLING a windows update that you have already installed (typically not a good idea unless you're having an issue with a newly installed OS update). If you've got a lot of these folders, deleting them can sometimes free up a bunch of space.
2) Page File: look at the advanced system properties and set C: not to have any paging file. It's not a bad idea to have 2-4GB virtual ram/page file set on another partition, but you can free up this space on the C:\ partition. Sometimes this will even benefit performance.
3) Trash Can: look at properties and configure the can independently for different drives. Turn it off on the C partition, or at the very least set it to %1 instead of %10. You can leave it at %10 of the other drive(s).
Of course, those suggestions are in addition to the Microsoft System Tool, Disk Cleanup, as well as the 3rd party apps others have suggested.