I have a new system (Toshiba Satellite A75-2131 running XP Pro) I'm planning to use my Echo Indigo IO for audio but need to connect two midi instruments to the system via usb if possible and practical. I've read that some midi to usb devices have had driver compatibility problems with XP.
I've used a Midisport 1x1 and a 2x2 without problems on XP. It's been reliable and the latency is as low as I need.
The only problem I've had is when changing the connection from one USB port to another. I have to teach my apps about this "new" midi connection when I do that. Solution: if you have more than one USB port, write "MIDI" next to one of them with a Sharpie pen, and always use that port for your MIDI connection.