You would need to ask HP, as the manufacturer of the device usually makes the drivers as well.
This has been asked on the HP forums it seems - http://h30434.www3.hp.com/t5/Notebook-Operating-Systems-and-Software/Windows-7-Driver-for-HP-ScanJet-3400C/td-p/233736
Have a read of the post as there appears to be one or two possible solutions which some people seem to claim to have worked for them.
Have a read and see what you think. As its HPs post on HPs forum, you are probably better off joining that to ask any specific questions. I don't have the scanner and haven't tried any of the suggested methods.