need help with windows xp on pc

hi there I need your help,
my father in law downloaded some games over the last couple of days and now his pc is saying that his pc mayb A victim of software counterfeiting and his copy of windows is not genuine.
he cannot find the software for his windows xp.
I have tried to do a system restore but nothing happens. I have scanned his pc for viruses there are none.
also now..the pc has no sound and the picture that he had for his desktop picture has gone too.
what can I do?
Jet88