Stephen, Have you installed a twain driver on your windows 7 operating system ?
Download from http://www.microtek.com/products.php?KindID=3&ID=35
You need to know if 32 bit or 64 bit version of windows 7
left click windows start icon left hand corner of screen
click on
Control Panel\System and Security\System
to see if 64 bit or 32 bit operating system
then download correct version and install by running file (click on file to run ) View in downloads off internet browser

DONT totally rely on WINDOWS 7 to auto detect your scanner and to auto download a driver as microsoft are stopping support for Windows 7