Hi Andrew`,
Welcome to Microsoft Windows Vista Community site.
This happens when windows mail is no longer set as an application designated to open contact files; to resolve this issue follow the steps mentioned below:
Step 1: Open your Contacts folder. Right click on any of the contacts and choose "Open With". The designated file should be "Windows Mail".
Step 2: If it says something different than Windows Mail you need to click on "Choose Default Program". Click on "Browse"
Step 3: The default location for the file is C:/Program Files /Windows Mail
Also check if Vista has probably changed the folder template; to resolve this issue follow the steps mentioned below:
1. In Windows Explorer, navigate to the C:\Users\(user name)\Contacts folder.
2. Right click on a empty space in the main middle pane of the window and click on Customize This Folder. (See screenshot below)

3. Under the What kind of folder do you want, click on the drop down arrow next to the listed template. (See screenshot below)

4. Click on Contacts to select it. (See screenshot below)
NOTE: Check Also apply this template to all subfolders if you want to change all subfolders in the Contacts folder to the Contacts template as well.

5. Click on OK. (See screenshot above)
Let me know if this worked.
Hope this is helpful.
Thanks and Regards,
Vijay K - Microsoft Support
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