Some time ago I came across the MigRecover Utility for Microsoft’s Windows Easy Transfer (thanks Calle Malmstrom for the tip!), It works great for extracting data from a standard USMT 4 store as well. There is also a version for USMT 3.
The syntax is quite simple:
MigRecover.exe [/i:<include pattern>] [/e:<exclude pattern>] [/v] <source MIG file> <destination path> [password]
So for extracting all files in the C:\MigData\PC0003\USMT.MIG store to D:\Tmp you would use the following command:
MigRecover.exe /i:*.* C:\MigData\PC0003\USMT.MIG D:\Tmp
For download and additional info: