jronan:
Look under File > Plot Setup, there is a driver for the HP Designjet 430. I had a HP 650C (since gone to plotter heaven) and had added a Directjet card which added an ethernet connection to the plotter. I don't know about a usb-serial pin connection, but the Directjet card made a much faster connection. You could also try saving plot (.plt) files (File > Plot...), then transferring them to your old computer that is connected to the plotter.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David