What printer model? Do you have a 32 bit or 64 bit version of Windows 7?
For 32 bit systems you can probably install the XP drivers.
For 64 bit systems you can download the XP x64 drivers from here, then unzip the files into a new folder. Next go to the Devices and Printers folder, Add a Printer, select the proper port, then select "Have a Disk" and point to the folder where you unzipped the files.
You can see the list of plotters this supports in the post here.
This forum is focused on consumer level products. For the Designjet you may have better results posting in the HP Designjet forum here.