Can't give you specific advice as you failed to include your PC model number and Operating System.

But, in General; if you do not use the "Safely Remove Hardware" to uninstall USB devices (except the printer) before you disconnect a USB device such as a memory key or maybe an IPOD Windows status can become corrupted and the next time you connect the device it won't be recognized.

Usually, when this happens if you plug in the device and then display the Device Manager usually in the USB section will be a listing for an unknown device.  Uninstall the unkown device and then disconnect the device and restart the PC.  After Windows (or Vista) completes it's startup, connect the USB device and usually it will be detected and installed.  If this works, always use the icon in the Sys Tray (Right click on the icon) to "Safely Remove Hardware" and "Stop" the device before disconnecting.

To access the Device Manager, hold the Windows key down and press the Pause/Break on the Keyboard.

Many thanks for your advice

Thought the system passed that through. I have a DELL Dimension 2400 and am runniing XP with all latest updates.. Problem began when I installed a new HP J6480 Printer. It was suggested I uninstall and reinstall printer. I did so, electing to not plugh in printer until full s/w install. I did and problem went away. Then 2 days ago, I decided to start using my  STAPLES hum again. That's when the problem resurfaced. I took it out, rebooted and problem remains. I took your advice with a thumb drive but problem remains.I then ran a program to look at all drivers for currency. It shows the 82801DB/DBM USB universal host controller driver to be out of date along with a few oter drivers. Could this be the problem? Any words of advice about updating drivers?

By the way, I WAS able to transfer pictures from my Canon camera during all this via the USB ports.

chris