If I recall correctly there are a few things going on aside from the nic's mac addr and the time.

I know that MS has been long aware that for example a PC with a Modem gets a "bogus" mac for example...

also I am sure they are using some of the same logic that a good random() gen uses to balance the numbers created, things like ticks + time + vectors such as a hardware port peek

also you can use id data from the PC BIOS and stuff like the hard disk serial number.

and I think there is a way that they mix and match the different parts of the data that adds to the uniqe ness.... I know that there are a few nibbles that are const for all GUID's

(Version ID for example)

I may be in error but this is what I seem to have read when I looked at GUID stuff in the past.