Prior to executing the next procedure check your USB devices - all of them should have cables not longer than 5,5 meters (including any of the passive extension cables). Make sure that they are all well plugged in and "sit" there firmly and check connections with any extension cables.
Run device manager:
1) Run "System Properties"
= [WinKey + Pause key]
= right click [My Computer -> Properties]
= [Start Menu -> Control Panel -> System]
2) Switch to tab [Hardware]
3) In section [Device Manager] click button [Device Manager]
4) In the new window [Device manager] check if any device or device type has a Exclamation mark or Question mark next to it
5) If so, extrapolate from there which device is causing problems by eliminating devices [in your head, of course!] without Question or Exclamation mark from the devices you have
If no USB device has a any of those signs then do this:
1) Go to menu [View -> Devices by Connection] in that same window (Device Manager)
2) Expand tree structure
6) Look for USB Universal Host Controller or anything else with the "USB" acronym within
Unplug your USB devices now, ONE AT THE TIME. After device has been unplugged Device Manager should automaticly refresh. Now plug in your devices, again ONE AT THE TIME.
1) Plug in device
2) Wait for Windows to recognize device:
A - message confirming that device is attached
B - message displaying the name/type of your device
C - message "Your new device is installed and ready for use" to popup
[Device Manager window will automaticly refresh]
3) Compare the actual name/type of device you just attached to "thing" Windows has detected. If it's match then it's... well, match.
4) Plug in next device and repeat this procedure.
If Windows does not detect your device, it will ask for device driver(s). Have your CD's handy. If, after you install the original drivers, Windows still reports "unrecognized device" go to device manufacturer's Web Site and locate appropriate updated drivers, if any. If this fails, then it's like asking "How many programmers are needed to screw in the Light Bulb?".
[The answer is "NONE. It's a Hardware problem."]
There is more possible problem. If you are using High Bandwidth devices like one or more WebCams, Scanner, etc. it is possible that you are "chokin'" your computer's USB controller. To check for this do the same UNPLUG-PLUG procedure with exception that after Windows detects device unplug it again, and plug the next device [so that at any moment you will have only one device attached].