If you move a drive from one PC to another, you can have permission issues also. This can also happen when booting to different windows OS installs on the same machine.
The permission issue is easily identified as shown in the above link (look at the screenshot). Just try it from your own addon directory.
Also, check that the folders are not read-only.
The issue with program files is due to an inherited permission from the program files directory. Thats why moving it out of there to somewhere it wont inherit the the restrictions on directory creation solves the problem. You can also change the directory permissions on the wow directory so they dont inherit permissions from their containing folder, then set whatever permission you like and tell it to set it for the current folder and all files and subfolders.
I believe i have seen mention of issues with virtualization after installing wow using its installer on a vista machine, but i dont remember all the details.
Its really just easier to move the wow folder out of program files under vista.
I think I even added a message box which lets you know about the permissions problem, though it might be in the version on codeplex, and not in the one on wowace.com. Not sure.
I run WoW on its own partition. I mount it as my W: drive.
Just thought I'd share that with everyone.
If you run it from Program Files under Vista, you will have issues which you will need to address, or it won't work. Fortunately you can just move it to another directory and run it from there without having to reinstall or anything. Run it from your desktop even.