Sorry I missed the model in the subject. At least it's not parallel
connected.

I think you should seriously consider replacing it once VISTA settles
down. Agfa say:

http://www.agfa.com/digicam_scanner_drivers/index.html

Agfa discontinued its DTP scanner product line (SnapScan, DuoScan,
Horizon and Arcus range) and digital cameras (ephoto, ActionCam,
StudioCam) at the end of 2001. However, driver & software downloads as
well as manuals remain available on this page.

There is an XP driver at:

http://www.agfa.com/digicam_scanner_drivers/scanner/snapscan_1212u/inde
x.html

Is that the one you used?

Try removing all references to the scanner that may be in Control Panel
/ Printers and Scanners; uninstalling any Agfa software you find in
Add/Remove Programs; unplug the USB cable from the scanner and from the
PC; reboot.

The install the Agfa software by locating the installation file you
downloaded/already have for your XP; right mouse click on it and select
Properties / Compatibility; set the Compatibility to XP with SP2 and
check the box ]X] Run as Administrator and OK your way out.

Either when the software installation procedure tells you or when it
finishes and you have restarted the PC, plug the USB cable into the
Scanner and the PC and see if it is recognized and runs with the XP
driver.