I need to start with some questions. Is your Plotter attached via serial port or is it attached via a network.
Assuming that you are attached serial port, make sure you have downloaded the lastest (appropriate) HP draftmaster driver. The installation program make have a "search for device" feature. Most PC have 2 serial port (port 1 or port 2).
If your plotter is attached via a network, Make sure you hvae download the lastest HP draftmaster driver. The installation program may have a "search for device" feature. Also, The plotter has to have an IP address. You may have to assign it a IP address. OR your HP printer installation program may dynamically assign it an IP address. Also try the Windows XP Pro "add printer wizard" and follow the prompts. It will ask you for a network printer and you will have to assign it a name and A IP address.
I hope this helps. Texaspete