I'm having the same kind of issue with my Mac/Parallels/WinowsXP...
I have three printers that all work fine in OS X attached to my Mac in the following ways:
GCC Elite XL 20/1200 laser printer - ethernet
Epson Stylus Photo 2200 inkjet - firewire
HP Photosmart D5160 inkjet - USB
After loading Parallels and Windows XP I get the following problem.
Only the HP shows up under the Devices menu in the USB side bar - but when I try to instal the driver in Windows i get this message:
"USB device you are trying to connect to the virtual machine is being used by another process. Wait 5-10 seconds, then try to connect the device again. If this does not help, find the application that blocked up the device and disconnect it manually."
huh?
I have downloaded the driver for the GCC and gone through the Windows set-up wizard but since it is connected to the Mac through ethernet - the only option seems to be setting up standard port with the TCP/IP address - yet it isn't communicating with the printer. The files i want to print are spooled - but I just get an error - as if it can't find the specified printer.
I haven't yet tried to load the driver for the Epson - as it looks like Windows is unaware of firewire devices anyway.
All of these printer are working fine in the Mac OS - so any help would be greatly appreciated!