The Visioneer OneTouch 5800 USB works fine in Windows 7, they just dropped support for it.
First, install the 5800 driver/software for XP (OneTouch version 3.0). The driver won't be recognized, but you'll need the software later.
Now, download and install the Vista/Windows 7 driver for the Visioneer 9650; it is close enough that it will work. Choose the unknown device in Device Manager, update the driver, tell it you will pick from list, choose Imaging Devices, scroll down to Visioneer, pick the 9650 driver.
Use the _old_ software "Scan Manager" (from OneTouch version 3.0, not 4.0) to scan from the scanner. You'll be able to pick that option in whatever software you use to scan.
Edit: By the way, I'd like to mention to VISIONEER -- if they ever stumble across this post -- that because of things like this, I will avoid their products in the future. It took me all of 30 minutes to get this scanner working on Windows 7 64 bit. It would take an intern engineer all of an afternoon to put together a working driver/software package for their legacy products that a normal user could install. Just put a disclaimer that legacy drivers are for the convenience of customers and are not officially supported. It goes to show how little they care about their customers.