This is the install software for the AcerScan 620U, which is the USB model. I wasn't able to find it anywhere on driverguide.com and when I tried to download the latest version off of the Acer website it didn't work, crashed my graphics programs. This one worked perfect.
How it works
This will give you disk1.zip and disk2.zip
Unzip disk1.zip to a folder called disk1.
Unzip disk2.zip to a folder called disk2.
Run the 'Setup.exe' file in the 'disk1' folder - halfway through it'll ask you where 'disk2' is. browse to wherever you've unzipped disk2 and that's it!