Reboot your computer with the drive plugged in, then at the first screen (which probably shows your computer's brand) look for an option that says "boot options".  It's most likely the F12 key.  Actually you can probably just reboot then mash the F12 key a bunch until that BIOS screen pops up.  Select the usb drive then hit enter.

When you're done, just reboot and you'll be back in Windows.