You'd be able to build something more cheaply (and/or more powerful) than a pre-built one. Just depends on what you want. Building isn't terribly complicated and you can find lots of guides or ask questions for assistance.
Depending on where you live, you can also take advantage of getting a student discount on Windows 7 Professional. I picked up Windows 7 Pro 64-bit for $30 back when the student deal for it was new, but it's gone up to around $60 now (and the US price, I don't think it's available in Canada). However, it'll probably be the upgrade version so you might have to do a little work around like I did and install it twice for the product code to register correctly.
If your school has access to MSDNAA you could try to get into that to get certain Microsoft programs.
Essay Pro