Fresh install of Win 7 Home Premium 64bit is just over 20gb. Move the page file to a separate partition/drive that 20gb gets a lot smaller.
Do some tweaking and get rid of unwanted stuff like EI8 and the 'games' that install with windows. My OS Partition is currently using 10.6gb, which includes drivers and related software. I have the partition set to 25gb in case I reinstall Windows for whatever reason.