A quick guide to make an USB Flash Drive bootable. This is usefull if you use Microsoft Windows installation images.
Open Command Prompt as an Administrator.
Type list disk (note the drive number of the USB flash drive)
Type select disk X, (where X is the drive number of the USB flash drive)
Type clean (delete all data from the USB flash drive)
Type create part pri (aka create partition primary)
Type select part 1 (to select the partition just created)
Type format fs=ntfs quick (to format the partition)
Now the USB flash drive is ready for a Windows installation image