HP Solution Center does not recognize the scanner anymore... We are operating on a Windows 7 system... the drivers had to be downloaded from the internet because the cd that came with the printer was for Vista... It worked fine for a month and then yesterday it didn't recogonize the printer nor the scanner... we reinstalled the drivers again... the printer works but the scanner is not recognized. (We have the HP Pavillion desktop...)

I had the same problem and solved it. I was using HP Solution Centre to run my HP Scanjet 5590 on Windows 7 Pro 64 bit without problems. Then I ran the scanner using Windows Fax and Scan, but didn't like it, so went back to HP Solution Centre - whereupon the scanner was not recognised.
After hours of reinstalling software etc etc, without success, I suddenly noticed four device icons on the left hand side of HP Solution Centre. These were labelled (when hovered over with the mouse) hp scanjet 5590, hp scanjet 5590 #2, hp scanjet 5590 #3, hp scanjet 5590 #4. Only the top icon was active, but when I clicked on any of the four it moved to the top.
Clicking on hp scanjet 5590 #2 gave me a working installation once more - problem solved. No doubt the multiple icons arose from my desperate attempts to try different USB ports.