If the repair-shop man ends up needing to reformat, which may be the easiest solution, he's either going to give you a pirated version of Windows (which is actually very common for repair shops to do, at least around here, as I don't know of any around here that don't pirate), in which case we couldn't offer you support, or if he has to reformat with a legitimate copy, you'll have to pay for at least a license key, which is going to be up there. What you're going to want to do is buy a retail disk because if you buy an OEM, it cannot be installed more than once (so if you ever need to reformat again, you'd need to buy another one!) whereas if you buy a retail copy of Windows, you can install it over and over on the PC with the same license key, and if you want to put it on a new PC, all you have to do is uninstall it from the old one and then put it on the new one! With OEM or just a plain license you can't do that.
So, essentially, if you don't have the recovery disk that came with your PC, the best bet is to buy one of those products I liked to.
