If you have good internet connection, then you can download your windows 7 version here (you still need a legit key ofcourse).
Then you can use Microsoft own tool to make a bootable windows 7 USB.
- Start up Windows 7 USB/DVD download tool when you have downloaded and installed it
- Select the iso file you downloaded in the Source field and click Next
- Insert your USB drive which you want to use (4GB is enough)
- Select USB DEVICE and click Next
- Select the USB device you want to use and click BEGIN COPYING
When it is done, you should have a working bootable Windows 7 USB