Running XP pro on a

Question

Running XP pro on a Compaq Presario Desktop AMD. Two USB ports on front of unit worked prior to reinstalling OS .Afterwards plugging in a flash drive that worked previously... Computer will Sometimes assign a drive letter .. but will not show the data on the flash drive. Sometimes will NOT assign a drive letter and indicates a problem with the installation of the flash drive. The Flash drive works properly on other computers. Any ideas ? Thanks Goose