I've had to tell a few people about this recently, so I thought I should actually write a blog post on this.  The options are very simple and easy.

• Buy Windows 7 on DVD and use or buy a 4GB (or larger) USB Stick and then make a USB stick bootable and copy the files to the stick.  Details on how to make it bootable can be found here - http://www.techmixer.com/install-windows-vista-from-bootable-usb-flash-memory-drive/.  You literally copy all the files from the DVD to the USB stick for this to work. To do this, enter this command into the run box (press Windows-Key + R) or a command prompt window - robocopy d:\ e:\ /s - this assumes that the DVD drive is D: and that the USB stick is E: - change them as required.
• Buy Windows 7 as an ISO from the Microsoft store (http://emea.microsoftstore.com/uk/) and use the Microsoft tool from http://store.microsoft.com/Help/ISO-Tool to copy this to a USB stick (as mentioned above)
• buy a USB DVD Drive (not my preferred option)

I hope this helps.

David