Here is how I fixed it on Vista 64bit:
1) Open WindowsExplorer and find the "addons" folder, then right/click it and choose "Properties"
2) Chose the "Security" Tab and click on "Edit"
3) Now choose each entry under "Group or usernames" once and put a hook in the first field "Full Control" under "Permissions"
4) Click "Apply" and VISTA should unlock all addon folders and files
This works perfectly for WinVista 32bit too. I tried on ultimate and home premium, both worked.
Fast, clean and you don't need to move your WoW folder!!