Uni computers will use Windows, but I had a Linux laptop(another operating system, different to both Windows and Mac) for the first two years of uni. I've since got a Windows laptop for reasons that didn't involve me making an active choice to have Windows, and I can't say that I've noticed any particular improvement in compatibility.
For instance, you don't need MS Word to be able to save documents in an MS Word format. I use Open Office (a free word processor that's available for Windows, Mac and Linux) and it gives you the opportunity to save in any file format you like. Similarly, MS Word also allows you to save in .odt format, amongst others.
One thing I will say with choosing a laptop is to go for one with a long battery life. There won't always be a socket available (especially in lecture theatres) and battery life does degrade over time. There's nothing worse than your laptop suddenly dying at an inopportune moment, like in the middle of a lecture if you type notes! The other thing is that as a geography student you simply don't need anything particularly powerful - the vast majority of what you'll be doing will be writing essays, reading journals and writing notes, which requires only the most basic of laptops, and certainly not a Mac!