yeah agreed
they could have also used a reg fixer which could have damaged the product key.
you need to get in touch wth them and make sure they used a legit copy of the OS.
you can just click the activation and try to activate ove rthe web. if all is well it should just work if its a dodgy copy or already registered it will tell you (you wont get in trouble)
you can call MS and tell them as long as you do have a copy of the os your self they will tell you an activation code to use.
if it has been reinstalled likley hood is that all your settings and email accounts have also gone.
or just refer it back to who ever fixed it