If you are looking for a simple GPS receiver that uses USB connectivity I have used this one before:
USGlobalSat 20 Channel BU-353 Waterproof SiRFIII USB GPS Receiver - RobotShop
It is small. Powered from the computer's USB port. Has a magnetic back so it can stick to the vehicle somewhere and waterproof so it doesn't stop working if it gets rained on.
It sends a text string through a virtual COM port. The string conforms to NEMA 0183 standard - if that means anything to you. It is easy enough to parse the string to determine longitude/latitude.