Scanjet 7400 not found by HP software

I have a user with an HP Scanjet 7400 which was installed and working fine on his older computer with XP Pro. I installed his new Dell Optiplex 755, also running XP Pro, and the Windows software finds and runs the scanner just fine; I can scan and save an image. The HP ScanJet Pro sofware doesn’t find the scanner. I’ve downloaded the latest drivers from HP, completely uninstalled the sofware, powered down and removed the scanner and cleaned the temp files and registry before doing a reinsall. Used msconfig to disable all programs at startup and reinstalled the HP sofware to no avail (twice). Any ideas or suggestions would be greatly appreciated.