>SetVolumeLabel() is as close as you can come. The API provides no >functionality to directly set the serial number. In order to do so, you'd have >to access a couple of ROM BIOS interrupt calls. Specifically, Int 21h Function >440Dh Minor Code 46h and 66h for FAT32. In order to do so, I think you'd have >to write a virtual device driver in C.

Yup. In fact, you would need the manufacturing firmware as well. When a drive is shipped, the firmware is changed to OEM customer specific requirements. In most cases this firmware would not allow a serial number to be rewritten.