Apple recommends a 20GB partition when installing Windows through Boot Camp for a reason. Windows itself can take up over 2GB before you do anything else, and once you install the (very much mandatory) updates and patches you'll be up around 3.5 or 4GB. Programs often take up a lot of disk space too-Office for the PC takes up several gigabytes if you install the whole thing. 5GB is just way too small for any Windows installation.