Unbelievable - it works (Apple BlackBook Core2 Duo, 2 GB RAM, XP SP2, Parallels 3186). 
What I did? After having installed MapSource City Navigator Europe v8 without any success I tried out new Garmin USB drivers: 
Goto Updates & Downloads -> Additional Software (
http://www.garmin.com/support/blosp.jsp). Look for your GPS device, I found under "Quest" USB Drivers software version 2.2 as of February 28, 2007. Voilà! 
I downloaded the .exe-file, installed it under Win XP/Parallels: 
Part 1) 
During the installation procedure I was prompted to power on my GPS and connect it to my computer. Unfortunately my Quest (as always before) was _not_ recognized by the system and so the installation could _not_ be completed. 
After quitting the installation I went to Windows in Device Manager and looked for yellow Question Marks or Explanation Points pointing to unsolved problems. Under USB I found an unknown device labeled with a yellow Question Mark. Right clicking on that  brought up a properties window. Clicking on the Drivers Tab, then clicking in the UPDATE or REINSTALL DRIVER Button started the drivers' installation process. Later I clicked "install automatically" (or so) and completed the installation. 
Part 2) 
I rebooted Windows after disconnecting my GPS and again installed the formerly downloaded USB-driver .exe file. After I was prompted to connect the powered-on GPS device Windows opened a window telling "new hardware found". After that I did what the installation program wanted me to do - and hey - suddenly my Quest was recognized and I could complete the installation process! Another Win-reboot and I was done! 
So far no problems or complaints, MapSource software works with my virtual Win like in my dreams. I would suggest you to try out some new USB-driver installation as pointed out above. 
Good luck, 
A.
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