If you are part of the ACM ($20 per year fee and is for students in a scientific, technological or computing course of study) you can get Windows 7 Pro for $20. Also, the ACM gives you access to resource materials and other interesting things as well.
MSDNAA gives you downloads of OS's and other things like Office Visio etc for about $20 per year I believe. I joined it with my school's IEEE and should get Windows 7 through that. If you are in school in STEM (Science Technology Engineering and Mathematics) area of study it's a very good option.