Make sure you do the various updates to Office 2008 after you upgrade to Leopard 10.5.
Office 2008 has been updated to Office 2008 SP 1, 12.1.0 and then in December 2008 there was another update 12.1.5, released 12/9/08 and is a critical update for Office 2008.

Even if you still have Office 2004 make sure you update. There have been several updates Office 2004 11.5.1, 11.5.2 and 11.5.3 from December 2008. These updates are critical updates for Microsoft Mac Office 2004

Office updates are pretty easy to check for and apply. Simply launch any Office application and click on the Help menu and select Check Update and the Microsoft AutoUpdate program will launch.

You are updated when you launch the Microsoft AutoUpdate and check for updates and there are no updates available.

The latest versions as of February 2008.
Mac Office 2008 – 12.1.5 (081119)
Mac Office 2004 – 11.5.3 (081112)

And always make sure you have the latest updates to the Mac OS. currently at 10.5.6.