I get this sometimes as well (running Vista Ultimate 64-bit).
What I do, and it's a pain in the rear, is that I turn
the computer off, then unplug the USB cable going to the
computer. I also turn the Pronto off with the power switch.
I then start the computer and when it is booted I reboot
it again. After the reboot I plug the USB cable back in.
And finally, after the USB cable, I turn on the Pronto
and put it in the cradle once it is booted.

I don't have this whole procedure down to an exact science
(is that even possible?) but it does do the trick for
me.

Noteworthy is that this problem doesn't occur very often
at all. :)

I hope this helps.