Try to google and download winDV (Small DV capture application) to see if the problem lies within premiere, but it looks like from what you described that XP doesn’t actually even see the device. Did you say that you tried 2 different camaras ?
Then, it’s either your Firewire card (I am assuming you have a card) or its driver. It might be worth buying a cheap (returnable) $10.00 card to see if that’s the problem. If it is indeed a card, you may also want to try changing it to a new slot to kind of reset things.
Finaly, it just could be a corrupted driver. You could try unintalling the driver through device manager and let xp reinstall it at start up.
Just for details sake, is the camera the only thing plugged in the port ?
Is this a Firewire card or on integrated port ?
Cheers,
Vince