While waiting for that definition, I can add that chemistry in general is mostly a Windows world. Almost every analytical instrument runs on Windows, most professionals use Windows. For your own use, then, I would recommend a PC because it will make transferring data and files more straightforward. Compatibility issues between Mac and PC are much less than they were when I was in school (my PhD advisor used a Mac and everyone in the lab used a PC; it was a nightmare to send him documents), but nevertheless, why bother with the headache? Certainly if you want to use instrument software on your own computer for working on data off-line, you'll probably need a PC.
As for what software, that will depend a lot on what MChem is, but Microsoft office is probably a must. ChemDraw is also very useful. If you are going on in the sciences, you may consider OriginPro, which is indispensable for data analysis and plotting. Also, you may want to consider getting photoshop and illustrator. Learning how to use those programs (especially Illustrator) was one of the best investments of time I ever made. Being able to make attractive figures and diagrams is a useful skill for presentations, posters, and manuscripts. I also use Mathematica frequently, although it may not be necessary for you. Note that there are open source versions of most of these programs if you don't have the money for the official versions, and a lot of them have very heavy student discounts.