just because its been used a certain number of times doesn't mean it wont work.
I upgraded a Samsung notebook running xp to windows 7 as the original xp backup on disc d had been corrupted & didn't get a reinstallation disk with the notebook, after breathing life into the notebook I found it ran great & decided to up grade the hd to a 500gb. As I bough a 3 license windows 7 disk I was happy to reinstall. After that in upgraded my desktop from vista to 7 meaning id used all 3 licenses but few weeks after I decided to upgrade the desktops hd from 147gb to 1tb & after some research I was informed on the microsoft forum that being USED on 1 hard drive meant i was currently using 1 license so as long as I don't try to run 2 pc's at once it makes no difference how many times I installed windows 7.
simply put, if I bought one license I could re-install as many times as I needed, 1 license 1 hard drive even if I decide to fit a different hard drive id still only be using 1 licence, but 1 license on 2 hard drives on 2 different pc's would void one installation.
having nothing to loose I fitted the 1tb hd & installed the windows 7 disc. 9 months later both machines are running even after installing & activating more times that the license allowed so I assume the forum members on the windows site were right.
of course some did hint that's not what windows meant by one license = one hard drive & that Microsoft had meant 1 license meant=1 activation & that I would burn in hell for all eternity if I try to install & activate it, but ive now installed 5 times & im not burning in hell & the pc's are all working & receiving updates.
if your sons installed version of windows is not running correctly & your thinking of buying a replacement xp installation disc I would say to try your own disc again first. But have windows not stopped doing updates & service packs for xp, I know they planned to do away with updates & service packs for personal use but keep them going for business use until 2017. Best check that out first before you do anything as its may be a better idea to buy a windows 7 installation disc & upgrade your system if it can support w7.
there's down loadable tools on microsofts website that can check if your system can run w7 but avoid up windows vista like the plague if you do upgrade as its crap & didn't work well at all.