This morning I realised my Outlook PST files had not been backed up since I upgraded to Outlook/Office 2007.  It seems that the backup addin stops working after an upgrade.

A quick Google search led me to this information:

Here’s how to get the free Personal Folders Backup tool from Microsoft installed and working in Outlook 2007:

1. Download the pfbackup.exe file.
2. Use WinZip or a similar file extraction tool to extract the files in pfbackup.exe, launcher.exe and pfbackup.msi.
3. Double-click pfbackup.msi and follow the prompts to install it.
4. In Outlook 2007, choose Tools | Trust Center and choose Add-ins from the navigation list on the left. At the bottom of the Add-ins screen, under Manage, select COM Add-ins and click Go.
5. On the COM Add-ins dialog, click Add and navigate to the C:\Program Files\Microsoft Office\OFFICE12\ADDINS folder. Select outbak.dll and click OK. When you return to the COM Add-ins dialog, you should see a new entry, Outlook Backup Addin, with its check box checked.
6. Click OK to close the COM Add-ins dialog.

Once you have the add-in installed, you should be able to choose File | Backup on Outlook 2007’s main menu and set your backup options.

It’s worth adding that if you’ve already got the addin installed, at step 3 you should run pfbackup.msi to uninstall the addin first and then re-run the installer to install it again.