Today, I try to install Windows 7 from USB for my friend, I searched on the Internet and found two ways to install windows 7 from a USB.
1. Using UNetbootin, you may download from here.
With this free tool you may format your usb to NTFS file system before burn your Windows 7 ISO file to the usb.
2. Most easy eay is using: Windows 7 USB DVD Download ToolIt automaticly format your usb to NTFS file system and burn the Windows 7 iso file to the usb .
Try to boot from usb first to make sure your computer has usb boot feature.
good luck !