This is not a huge space maker, but it gives something. There are old installer files in your hard drive that can take up an enormous space on some computers.
You left out a bit of important info. How large is your hard-drive, is it split between different partitions? You may just need to buy a larger hard drive, between constant Windows updates and applications getting larger and larger, a small drive just won't work well anymore.
You can also turn down the space used by System Resotore in addition to Tig's recyble bin suggestion.
Open Windows Explorer (right-click the Start button, select 'Explore all users'), and navigate to the Windows/Installer folder. Right click and check properties. It will show the size of the installer folder which may be any size imaginable. Let us know the size of the drive and the size of the installer folder for advice.