It seems that many people are still getting the 'USB Device not Recognized' problem from time to time. Unfortunately there is no easy fix.
Firstly, it's recommended that you make sure every one of your motherboard drivers are completely up-to-date, but you probably already knew that.
In XP, after a USB port isn't used for a long while the USB driver built into the OS sets the OHCI controller to suspend. When you plug a device back in, it can sometimes fail to 'wake up' properly. This problem was supposedly fixed in SP1 but there is a related registry hack that may still be able to help. (Only edit the registry if you know what you are doing!)
Run regedt32.exe and navigate to;
HKEY_LOCAL_Machine\SYSTEM\CurrentControlSet\Services\Usb
Create the Usb key if it doesn't already exist. Under this key, create a new DWORD value called DisableSelectiveSuspend and give it a value of 1. Then restart the PC.