Hi Cop mum, unfortunately, there's not much you can do if you have to reinstall windows unless you know some tricks regarding the registry, that I can't talk about as it would jeopardize the whole project.
The point of the project is to give you an extended play with all the games, (and applications) that is until the dreaded problems that mean you have to reinstall windows or do a full format, (which destroys the registry, though if you're lucky enough to be able to just repair your windows installation the registry should remain intact. The worst that can happen though is a hard disc failure. There's no reason to worry though, especially if you keep your machine in tip top health. Plenty of cooling to ensure the computer doesn't overheat, and regular clean ups of useless stuff, preferably manually as opposed to using a proprietary cleaner. Also don't use uninstallers if you wish to keep the registration details in the registry.
I retired my oldest machine once I'd got around 150 games from the giveaway project as it was full of other 'bought games as well anyway and was getting rather full. I've added new fans and HDD coolers to ensure the drives don't overheat. :) I never add anything else to that one apart from new scores when I decide to play a game, lol.
The computer is worth far more for just the games and some of the applications that I find useful but not enough to want to buy the applications than the hardware it contains. That's one of my ways of keeping the games for as long as possible, though my favorites I will buy anyway as i want the ability to be able to transfer them from computer to computer etc.