What does "x15-65908" mean?

[Wicky12]

A close friend moved to Australia recently.
He left with me a desktop top notch PC and a few disks.
One of them is marked: "Windows 7 Ultimate x86 x15-65908" on its box.
I believe this is installed on the machine.
The "x15-65908" makes me reluctant to use the machine.
My question:
What does "x15-65908" mean?

Thanks.

 

[HammerHead]

Media Fire

Appears to be a media fire download designation.