No available space on bootcamp partition.

I just created a 20GB partition in bootcamp, and installed windows 7 onto it. Then I installed the bootcamp thing on the mac os install disk, and then downloaded google chrome and malwarebytes anti virus.

So the entire reason i partitioned my hard drive was so that i could install the free version of solidworks that is provided by my school. So i went to install the program (5.4 GB) from my external hard drive and apparently there is only 1.4 GB of free space left on my windows partition. I emptied my downloads folder and checked the uninstall-program thing in the control panel and the biggest application that i have is the NVIDIA display control panel thing @ 135 MB.

Does anyone know what happened to the 20GB i expected to be able to use, I know that windows 7 takes some harddrive space but like 18GB seems like a bit much.