When installing a USB printer for a client recently I found that the installation failed with a "specified port is unknown" error. After much mucking around I found that the cleanspl.exe program had been run on the PC. One of the side effects of cleanspl is that it deletes some registry settings needed to install USB printers.

To fix the issue run regedit and go to the following location in the registry

HKEY_LOCAL_MACHINE/System/CurrentControlSet/Control/Print/Monitors

You will need to add a key call USB Monitor, and under that add a string value called Driver with the data field set to usbmon.dll.

Reboot the PC and your printer should be detected. If not, try uninstalling the printer drivers first (a lot of manufacturers offer an uninstall option with their printers).