[entwisi]

There are some 'genuine' windows license keys that are being identified as dodgy. There is a phone number for Windows activation, If you are absolutely sure of its provinence then ring them and tell them its life history and they should be able to issue with a new number.

You do need to know where your dad bought it from though. If its a large store like PC world etc then you will be fine, if it was some bloke on a market somewhere then probably( but not necessarily) not.

The workaround Kipax mentions is a way that confuses the windows genuine check tool into thinking its legit even if it isn't.