The Chinese black market has started selling copies of Windows 7 on 8GB USB flash drives for only $14. The Chinese have been selling these Window 7 USB drives long before the official release in October.
Microsoft at one time had the bright idea to do this but, why they didn't pursue it beats me. Installing Windows 7 from a flash drive is significantly faster than installing from a DVD.
Although, keep in mind that purchasing pirated copies increases the chances your purchase will include malware preloaded with the installation of the operating system.