BB's Suggestions are excellent, also, check in the bios that the parrallel port is set to Bi-directional. Also, check your power settings. It seems that the laptop may be changing state when plugged into ac, ie like docking. You may have to use the system control panel to see what drivers are used in different states.

Otherwise it is tough, XP does block direct access to ports, I had trouble doing project with serial port communications. Had to use HyperTerminal workaround

Let us know how you go.