Mounted image, drive letters changed

Hi,
Can an antivirus program run on a mounted drive image?
I mounted a Norton save and restore image of my drive on an external hdd to copy some folders to my computer, the progress window said it was transferring some NOD files and then hung with 5 seconds remaining for about 10 minutes. I didn't have any NOD files in this folder so thats why I'm asking.
(NOD was installed when the image was created now I have AVG.)

I canceled the process and the application froze, tried to bring up task manager and my task bar disappeared and control+ alt+ delete didn't respond so I had to reset with the external hard drive plugged in because it was still running. The ext. hdd has several partitions and now the drive letters have changed.

Can I simply change the letters back to what they were, using disk management, so that my backup program can access the proper drive?
I am rather puzzled with this (actually I am puzzled most of the time) and don't want to create any other problems.

Thanks for the help
XP SP2
1GB RAM