First of all, let me be clear that I have read most of the available posts I was able to get my eyes on.
So, I'm still feeling disappointed by HP not having proper drivers for ScanJet 3400C and Windows 7 32, not to speak about W7 64.
For personal reasons, I have refurbished my home main computer with a 2TB SATA HDD. There, I have installed Windows XP SP3 32 bits, to host some applications I couldn't use in other O.S.'s and was foreced to keep using. So I have installed the proper drivers for ScanJet 3400C and everything is working fine... under WXP!
I have another install with W7 64. There I have just installed the fresh new Batman Arkham Origins, which happen to run only in W7 and up. No way to install the drivers for my scanner. I have read on using a virtualized WXP on it, but it is out of my purposes for this machine.
After finishing install, W7 managed to build a menu for multiple startup so I can still use the scanner... although in WXP only!
On the same machine and disk, I have a third install with XUbuntu 12.04 LTS. After it ended the installation process, it build its own multi-startup menu, so I keep being able tu use WXP and W7. Moreover, I didn't need any special driver to use my 3400C. Simple Scan is perfectly capable of using it "right out of the box".
Hence, I have three OS's in my system and can use the scanner in two of them. Not bad but far from perfect.
I hope somebody in HP reads this conversation (not just this post) and gets the message: We all want a proper driver for W7 and our excellent 3400C!