Canon do not support the LiDE 20 on Windows 7 (no Windows 7 driver).
However, you will be able to use it on Windows 7 with a scanning application called "VueScan" from here: http://www.hamrick.com/
Download and install the trial version of VueScan just to make sure that it works with your scanner. If it works, you will have to buy it if you want to keep using it after the trial period has ended. The Standard Edition costs $39.95