We use the PCI 2-Port serial card (P/N 50000665-01)
Is this hardware XP compliant? Or more to the point, is it supported under XP?
The part whose part number you’ve listed is an AccelePort 2r920 PCI serial card.
Here is a link to the Windows XP driver page for that product:
http://www.digi.com/support/productdetl.jsp?pid=1324&osvid=57