If you are going to buy a Mac wait until you get your confirmation letter from the uni/UCAS as you then get a 15% student discount in store and online! (On Macs this can be a saving of up to £300!)
Personally, I'm waiting until September to get the latest MacBook Pro/Air that's rumoured to be released soon. That way I can get the student discount, and go to uni with a nice shiny new laptop.
Also, if you use a program called Bootcamp (which is standard on all macs) you can install Windows onto the Mac, so you don't need to worry about Windows only programs if your course has them.
Just a little tip, but like it's been said before go for Macs always! They keep their value if you look after them (I've got a unibody Mac from late 2008 that I'll be selling and hopefully get around £500 for it) and buying a new MacBook Pro in September will mean the machine will last me my the four years of my course!
Windows laptops (unless you get the very high end ones) are clunky, get viruses, don't hold their value and don't last as long.
Once you go Mac, you never go back!