Microsoft Windows
FREE
Ask your teacher, school IT lab or student bookstore how to obtain the free Microsoft MSDN Academic Alliance version of the software.
http://msdn.microsoft.com/en-us/academic/default.aspx
If you can download the iso image, here's a good free iso burner in case you need one.
http://download.cnet.com/Active-ISO-Burner/3000-2646_4-10602452.html
DISCOUNTED
Check your college book store for the Student edition of Microsoft software, or buy online here.
http://www.journeyed.com/select
Microsoft discounts for students:
http://www.microsoft.com/student/en/us/software/windows/default.aspx
If you are not a student and want to pay less than retail you can find Microsoft software on eBay.