Exactly the same amount it takes up on any other hard drive
Sorry couldn't resist that hehe. Windows 7 depending on other software or updates will take up roughly 15 to 20gb space on a hard drive (64 bit home version).
I found a clean install of Windows 7 Pro took up about 24Gb of a 128Gb drive when new (I added some additional Windows features like IIS for web development work). I thought this drive was ideal because I'd be installing all games and programs on a larger drive (and did so), and I also move all my email and Library folders to another drive for backup purposes (documents, pictures and such).
However, after two years, 90Gb is full. I cleaned out all temporary files and bloat but it didn't make a huge dent. The problem is that a lot of games and programs dump into the "C:\Users" directories, and after a long period they can bloat (along with all the various Visual Basic run-times and such, and anything else that's hard-coded to install to the C drive). As a result, I bought a larger drive for Windows 8 in case that eventually starts to suffer the same fate.