You want to have HP send you a free upgrade to Windows 7 based on the serial of a laptop you don't own. I'm not even going to start on how illegal that is...
Not just illegal but immoral. The OP is suggesting shafting some other poor user by stealing
their entitlement to an upgrade.
Actually it is illegal. You admit this fact yourself (marked in bold above). Your assessment of the situation is technically correct, when saying that he is shafting some other user by taking his entitlement to an upgrade, but that other person has already paid money
for the right to that upgrade.
The OP has taken that entitlement, and thus has stolen something someone else has paid for.