Before you make your USB bootable first step you need to obtain the windows Operation System core file and save it on your computer even if it is from DVD content. Copy whole content from the disk and keep it on your computer. Then you need minimum of 4 GB empty USB pen drive, then simply follow this below steps.
Steps: How to make bootable Pen drive using command prompt?
Step 1: Insert your USB on your computer Click Start button type cmd and right click on it click run as administrator now the command prompt will appear.
Step 2: Type diskpart ->enter list disk –>enter select disk 1 chose the USB disk number correctly you can recognize by its size. See below image my usb disk is 15 GB so I selected Disk 1.
Step 3: Type clean hit enter this will clean up your USB.
Step 4: Now type Create partition primary ->enter Select partition 1 ->enter Active
Step 5: Now type Format fs=ntfs quick hit enter. It will take few minutes to make your USB bootable.
Step 6: finally type exit after format is complete.
Note: Do not remove your USB disk while formatting it, if you disconnect your USB while formatting your USB may corrupt.
Finally copy your Operating system file in your USB without any subfolder like this below image
Also Read 4 easy ways to make hard disk clean up