This discussion kind of borderlines the warez policy, since you talk of having the same OS installed on 2 different PC's (only one of which belongs to you) and that violates Microsoft's end user licence. The reason you are getting that message should be obvious: your registration number is not legitimate, and Microsoft's server recognizes that it isn't.
Closed.
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Intel Core 2 Duo E6600 | 2 GB PC6400 DDR2 RAM | GeForce 7900 GTX
2.5 TB Hard Drive Space | Windows XP Pro SP2
Official betatester of PCSX2 and Co-owner of General Emulation