If it's Windows XP I don't think there is going to work System restore, because I don't think he did any Restore point... Anyways if you didn't get the CD, search Online for a Windows copy(I mean torrents and all that stuff). Then burn it and install it. On the back of your PC you should find your license key and what kind of windows you have. You don't have to buy new copy. You have license and it should be enough to reinstall them legally. Laptop sellers(big ones like HP) can't sell laptops without OS. And of course don't forget to find drivers(download them from HP) BEFORE reinstall and put them in USB flash or cd... You can find your exact model on the back too. If you don't find where to look, in HP web you'll find where to look exactly(there is a note). After reinstalling windows, install drivers.