You could try this, there is a free trial http://www.hamrick.com/
NB it probably wont work with existing Mustek software, but probably Win Fax & Scan
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The original version of this scanner (not the 600, 1200 or 2400) is said to be compatible on the Microsoft compatibility site, but does not seem to work, even though Vista 64 bit drivers are supplied.
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You could try this, there is a free trial http://www.hamrick.com/
NB it probably wont work with existing Mustek software, but probably Win Fax & Scan
I had contacted Mustek Technical Support Dep regarding drivers for the ScanExpress A3 for windows 7 (64 bit) and they kindly gave me a down link ftp://ftp3.mustek.com.tw/pub/new/driver/0\_Old%20Products/ScanExpress%20A3%20USB/ which I had downloaded the directory > Windows 8.1x32 8x32 7 vista XP and worked.
This is the a copy/paste of above link page: ftp://ftp3.mustek.com.tw/pub/new/driver/0_Old%20Products/ScanExpress%20A3%20USB/
The drivers do not come with a software user interface to scan but on inquiring again Mustek Tech Support Dep stated
"You must use a TWAIN compliant software to acquire our scanning TWAIN interface. Like Photoshop or MSPaint ,
Best Regards,"
From installing the driver on my laptop with windows 7 (64 bit) I have tested using Photoshop with its Scan TWAIM interface and it works.
I found this blog entry and it works!
http://www.paulstenning.com/mustek-scanexpress-a3-usb-scanner-early-version-on-windows-7/
mustek scanexpress a3 usb driver windows 7تحميل
I was able to get it to work. Here is what I did:
Step 1: downloaded the Vista 64 bit driver from www.mustek.de driver site (not available on standard site). It is called web64.exe. Who knows, it may work with the 32/64 bit version supplied on the standard Mustek site.
Step 2: VERY IMPORTANT: disconnect the scanner USB cable. Make sure all previous versions or attempts to install the software are uninstalled.
Step 3: right click on the web64 program and tell it to "troubleshoot compatibility". On the second selection, tell it custom, (or use the non-default settings), then select the option that the program worked under Windows Vista Service Pack 2. Tell it to "test" the program. At this point, the program will launch and install. When prompted DO NOT REBOOT, but cancel and end the install program. Return to the compatability program and tell it it was successful. End the compatability program.
Step 4: Reboot. Plug in the scanner USB cable. It should now be found and installed. Scan as normal using the included programs or other programs.