I think storms initial idea might be the way to go, you may even need to disable on board com/parallel ports in the bios, really free up the resources and then the card may find it's place.
I don't think ISA cards and Windows XP were ever really expected by microsoft to be used together, so there can be issues.
Corm
Edit: as a side note, how much ram in that system?, PII 350 is only just above Microsofts recommended specs for Windows XP so it will run a fair bit slower than 2000.