usb devices

My Dell 4300S dimension computer will no longer recognize my usb scanners, hp 44570c and Smart photo s20 or my lexmark x83 all in one printer, scanner and copy machine. I have followed all of hp and dell’s suggest with no positive results. “delete sofeware and re install ect, chage usb ports , use a powered hub ect. However my usb Zip dvd cd write still works , it appear that the usb ports work, but the system does not see them as connected.

bill