Have you looked at the permissions settings of the PMAIL directories you installed to on Vista?

Right click on the folder names in the explorer view and right click on it  and select properties then select the security tab from the displayed dialog box. Select your user name from the "Group or User Names" list at the top, and see which permissions are allowed and/or denied in the list at the bottom of the dialog.

 
You can do the same on your working XP machine and see if the settings are exactly the same or not.
 

I've just had to change my computer and now have Windows Vista when previously was using XP. With XP when setting up a new user ID on a different computer and restoring my backed up mail, I would simply copy all the files in the e.g. PMAIL\MAIL\admin folder into the same folder on the new computer. I have managed to setup Pmail successfully in Vista, but it will not accept my backed up files following the procedure described above. I've tried it again on an older machine using XP and it works no problem. All I get is the New Mail folder (but I can't delete or move any mail from it). I can set up and use new folders, but can't access any of my old folders. Does anyone have any suggestions as to what to do?

This sounds like some sort of permissions problem - although I have little experience of Vista myself.

Do you have full rights to the \PMAIL\ directories?

As far as I know I have full rights to the PMAIL directories. At least it's something I've done many times before when transferring my email from one computer to another and had no problem. I know for total certain that it's vista that's the problem, but there's nothing else different that I can think of - I'm using the same version of Pmail and following the same procedure as I've done before (and tried it again with XP on another computer since and had it work fine...).