Hey guys I'm new to the forum and came here just to check for a solution to this problem. However, even after trying matty's solution I still could not get Messenger Plus to import the skins. After meddling around for a while in VISTA, I managed to find a solution, so just to share with everyone - in case matty's solution doesn't work for you, follow his instructions till
"Click the Security Tab", then Click on "Edit" (to change permissions) > Click "Users ([User name]\Users)" > Check the box under "Allow" for "Full control" and all the boxes should be automatically ticked. > Click "OK" and accept any additional permissions required etc and you should be on your way.
I guessed that this was due to Vista's excessive requirements for permission if access was not granted to the User, and turns out I was right.
Hope this help all you Vista users out there!
Other OS users could TRY it of course - it might just help!