USB device won’t work/not found

I have a scanner that requires USB hook-up. When I load the software my computer won’t recognize the scanner. I checked device manager and I don’t see my hardware listed. I do get the yellow exclamation point for the unrecognized device. There are no hubs listed under the USB controllers. I have a digital camera that I’m having the same problem with. I’ve never used USB devices on this computer. It’s an HP Pavillion, WindowsXP home, 500MHz, 256 RAM, Intel Celeron.