An easy way to propagate the binary 'd be to run it on win startup, do checks on pre-determined interval for removable drives. If the drive has FAT32, copy self.
When attached to a machine later, there has to be any kind of misleading (like, say with icon of a folder, unless extensions are shown) so the user clicks to open it and the binary in turn copies itself to that machine and executes. With w7, I guess autorun is a bit more harder than previous versions.
Actually, that's what I did with my spybot in autoit.