There are severe web search issues going on with the planet.
First post found when you do a web search http://www.sevenforums.com/drivers/59069-installing-visioneer-onetouch-8100-scanner.html
The referred description is not clear enough, I think. On my system (Windows 7 Enterprise 64 bit) I need an additional step:
Install the PaperPort software shipped with the scanner. The scanner monitor software (windows tray) will find the OneTouch 8100. Now you can see two scanner devices (whereever you can choose a scanner): The 8100 and the WIA driver for the 9420. Only the 8100 works!