I am having a problem very similar to ebr's, and I agree that it seems to be a lower level issue. I am on Vista 32, and prior to connecting the HG20 for the first time, I installed both the Digital Video Solution and the ImageMixer disks that came with my HG20. The DVS disk only installed ZoomBrowser - presumably because the TWAIN driver only installs with XP.
After initially connecting the HG20 via USB, several devices were installed. I then attempted to open My Computer to see the new drive(s), but Explorer hung and I had to kill it. I eventually had to reboot altogether, because I could not get Explorer restarted. After the reboot, I had difficulty even getting the HG20 to install again, but after swapping USB ports, it was once again recognized in device manager as "Canon VIXIA HG20 USB Device" followed by a subsequent entry labeled "Disk Drive". Upon opening My Computer, the drives letters for these devices were not there.
On my PC, drive letters 'C' and 'D' are assigned to my HDD, and 'E' is assigned to my DVD drive. My PC also has a built-in 4-way card reader, and Windows assigns letters 'F' through 'I' to those devices - even when there is no media inserted. Per the advice I found on the Canon FAQ, I manually changed the drive letters for the card readers to 'L' through 'O' to free up some consecutive drive letters after the DVD drive, but even after a reboot, the problem still persists. Windows recognizes the HG20 as USB disk drives, but will not mount them and assign drive letters in Explorer. They are not present in Computer Management either. As with ebr, I am unable to populate the volume info for either device - though I don't have any driver error messages.