Is it possible to install and run Windows7 on a USB Flash Drive?

Is it possible to run Windows 7 from a USB drive?? USB drive with 120 GB are available, so storage won't be an issue, and we just have to change the boot device priority. So all i wanna know is, is it posssible to install & run windows 7 from a USB drive?

I had tried it on an external Hard-Disk, (external HDD and Flash Drive are not the same, but similar) and it didn't work. It just installed some files after the computer rebooted for the first time, it said "disk read error".That means Microsoft has blocked the OS somehow from installing in a USB drive (portable storage). If you find a way though, let me know too. I would like to know!