.

[old_death]

The user ID is partially calculated from your hardware configuration and should not be identically on 2 computers, except if they never run both at the same time. You might have a look at the registry if the GUID is stored there...

[skinvista]

GUIDs are stored in /Data/Profile.xml, not the registry.
(The value is viewed at >Tools>Edit Profile>Certificate.)

[raspopov]

No need. GUID is just an identifier used for internal purpose of p2p networks only. Every node (i.e. computer + network card) must be unique.