I just looked at the footer of one of mine:
Code:
63 6F 6E 65 63 74 69 78-00 00 00 02 00 01 00 00   conectix........
00 00 00 00 00 00 02 00-0B 0C DA 98 76 70 63 20   ............vpc
00 05 00 03 57 69 32 6B-00 00 00 00 80 00 00 00   ....Wi2k........
00 00 00 00 80 00 00 00-10 41 10 3F 00 00 00 03   .........A.?....
FF FF ED 72 9B DA 91 63-55 AF 11 DA 80 C6 EA CC   ...r...cU.......
5E FC 6D 16 00 00 00 00-00 00 00 00 00 00 00 00   ^.m.............
00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
00 00 00 00 00 00 00 00-00 00 00 00 00 00 00 00   ................
The "Unique ID" field is underlined. It doesn't look like any HD serial number I've seen before, and I'm all out of ancient DOS utilities to test it with (this is a DOS VM), but I suppose it's possible that the presented serial number is derived from that somehow.

Be very careful if you're going to try changing it that you don't have any differencing or undo disks associated with the one you're changing -- the Unique ID is definitely used for tracking those associations.

(No suggestions on tools btw; I cut that off the VHD with "tail -1b original.vhd > footer.vhd" (requires one of Cygwin, SFU, tools for SUA etc) and opened it up with "debug footer.vhd". Going back the other way would be harder.)



Edit: I just found a suitable DOS tool -- the hardware serial number reported in Virtual PC is blank. If there's a way to change that then it'd probably be a new setting in the .vmc file, but I can't find any documentation on the format of that file.