my c:\ drive was 100gigs and the operating system (vista) showed to be 36.8gigs...all software on the c drive...
I reduced the c:\ drive to 70gigs and now the operating system shows 15.8gigs...all software.I have not uninstalled any software.
Why the size change for the same software ?

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Answer from Richard Rost:

Windows creates a "swap file" which it uses to virtualize additional memory. This swap file is dynamic in size and may grow or shrink based on the amount of available hard drive space.

In addition, your Recycle Bin may grow or shrink to a percentage of available disk space as well.