Not too long ago, I partitioned my mac so I could put Windows 7 on one partition for gaming. I was dismayed to discover that simply installing Windows took over 20G. I've done some digging to figure out where all my space is going, using WinDirStat. One big space eater was hibernate, which I have since disabled using this how-to guide. Furthermore, even though I have 8G of RAM on this machine, Windows was setting aside a huge pagefile.sys. If you have a lot of memory, as I do, you can disable or limit virtual memory.