To do that manually you need to have having a Windows 7 Install Disc. Insert the disc into the DVD-Rom then go to Start > Run and type cmd to start a Command Prompt and you need to plug the USB drive to your computer. Let's say that the DVD-Rom's assigned drive letter is D: and the USB is G:.
First you need to format the USB drive. You can do this from My Computer. FAT32 is recommended as it's faster then use the Command Prompt and type these commands:
xcopy D:*.* G:\ /E /H /F
Wait for the copy process to finish then browse to the boot folder on the USB drive and type the following command:
bootsect.exe /NT60 G:
Remove the Install Disc, restart the computer and boot from USB and that's it.
