Some of the USB printers I've worked with have a conventional parallel port with a connector that converts the parallel connection to a USB plug. If this converter isn't working correctly or if a proper driver is not installed for it, then a USB printer port won't show up. The printer may come with its own drivers for this converter--especially if you're dealing with a printer that doesn't use PCL or PostScript. I owned a Hewlett-Packard LaserJet 1000, for instance, which used a proprietary rendering language (the graphics were rendered by the printer driver on the PC and then streamed out to the printer), so it wouldn't work or even register a USB port without its own specific drivers.
If you have those installed, or if the printer is a more generic model that works with plain-vanilla PCL or PostScript, there may be something wrong with the way the USB ports are configured in the system--a much larger issue. In a case like that, one way to force the system to redetect the available USB ports is to disconnect the printer, boot into Safe Mode, delete all the USB devices in the Device Manager hardware tree, reboot normally, let the computer redetect all the USB controllers, and then try re-adding the printer.
This was first published in August 2006