Har:

One other thing to make sure of –  is to make sure you're plugging straight into your computer and not through a USB hub.

Good catch, Har, and thank you for pointing out something that I'd completely forgotten!!

I have a nasty suspicion that this may be an example of a rare USB problem we have with a small number of XP systems.

Nick, I can't say I'm terribly surprised by this either, and it makes perfect sense. I've seen a number of older computers that were supposedly USB2.0 compatible, but weren't. In my experience, it affects most if not all USB2.0 devices and not just Eventide pedals. The only solution I've found is to either add a newer USB2.0 PCI card or replace the computer, neither of which is a feasible solution for most musicians. Unfortunately, about the only thing to suggest is to try a different computer. Thanks for helping to explain this fortunately uncommon situation.