Hi,
It seems that the driver for this model is unavailable on the manufacturer's website.
You can contact Acer support for assistance:
http://community.acer.com/t5/Monitors-TVs-Projectors/AcerScan-620UT-Windows-7-Driver/td-p/23347
Hope this information is helpful. Please do let us know if you need further assistance with Windows, we’ll be glad to assist you.