Instructions for the Acer (Benq) 620ST flatbed scanner.
How to get it to work in Windows XP SP2

This in the instructions for installing Mirascan 3.42 to Windows XP.

This is the User Manual for the AcerScan 620ST (about 3.8MB)

This in the instruction on completely removing MiraScan from Windows XP. This may be necessary if you screw up the first try.

To get the Transparency mode to work in Windows XP SP2, rename the file: C:\Windows\Twain_32\Mirascan\NT5Supp.sti. to NT5Supp.sti.csm. (Add .csm to the end of the file name).
Do not remove or move that file. Only rename it. Windows XP SP2 will replace the file from cache if you do anything else with that file.

Be sure that the transparency lid is plugged in and the black cover in the lid is removed. You must use the Transparency frame(s) supplied with the scanner. See the Parts link below to purchase the transparency frames.

A good Guide for using MiraScan Software. This guide is really for the Acer Scanwit film scanner, but works for the flatbed scanners also.
The Advanced MiraPhoto Guide

Parts for Acer (Benq) Flatbed and Slide scanners.
I just checked this link, all parts for the scanners have been removed.