Installing Windows on a Ubuntu 12.04 laptop

First check the laptop manufacturer's website to see if they have Windows drivers for that model because without them it's pointless installing Windows on it.

If they do have Windows drivers (and they most likely will have), your best plan of action is to buy a new hard drive for Windows and store the existing hard drive safely away somewhere, clearly labelled as being the original hard drive for that laptop model. That will ensure that you are able to return the laptop after 3 years in the certain knowledge that it's exactly the same configuration as when you took temporary ownership of it. That's what I'd do anyway in that situation.

Just buy the smallest hard drive you can get hold of to make it more affordable, you only need it for 3 years. It will always come in handy as a storage drive inside an enclosure when you return the laptop.

You could install Windows on a separate partition on the existing hard drive, giving you a dual-boot setup, but I don't advise that if you've got to return it in it's original configuration as setting up a dual-boot arrangement has the potential to screw things up and you'll be left with the unenviable task of sorting it out before you hand it back. Shelling out for a new hard drive from the start is the better option in my opinion. Buying one online will get you the best price compared to a physical shop.