I have the same problem with my Acerscan 620U flatbed scanner which I have used for years, even with XP home SP2 & 3, after I reloaded the Operating system on the same machine, (after a crash) It would not work properly, so I searched for new drivers and softwareafter not finding anything useful here, I went to driverguide.com and found 2 files for the 620U. A zipped file containing the original (Win 95-98) install disks - 2Megs, (pasted from Driverguide);
This is the install software
for the AcerScan 620U, which is the USB model. I wasn't able to find itanywhere on driverguide.com and when I tried to download the latestversion off of the Acer website it didn't work, crashed my graphicsprograms. 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.
Runthe 'Setup.exe' file in the 'disk1' folder - halfway through it'll askyou where 'disk2' is. browse to wherever you've unzipped disk2 andthat's it!
I also downloaded a 14 meg file "mirascanv403u10.exe" (<a href="http://members.driverguide.com/driver/detail.php?el=1&driverid=128406">Acer AcerScan 620U Driver</a>) which is supposed to work with W95-98, ME, W2000, XP (with mixed reviews)and Mac-OSX, I haven't tried either of these yet although I will try the Mirascan file first. For many users the difference may be that when setting up an XP machine you have the option to use a FAT32 file system or an NTFS partition, it may be that the software doesn't work with the NTFS partition/filesystem.
If you send me a request to firstname.lastname@example.org I will send you either or both of the files as an attachment until Jan 10 2009 then I delete them, (or you can get them from Driverguide) I'll also let you know what worked for me.