The Perforce Plug-in for Microsoft Office (P4OFC)
The Perforce Plug-in for Microsoft Office (P4OFC) allows documents created using Microsoft Word, Excel and PowerPoint to be easily stored and versioned in the Perforce SCM System. Both administrators and developers can now access an easy to use and powerful document management tool. P4OFC's one-step installation is accomplished by running a Windows installer program that does not require any additonal setup or configuration. P4OFC adds a simple "Perforce" menu to Microsoft Office applications so that you don't have to leave a familiar environment to bring your documents under Perforce's control.

P4OFC can display differences between any two document revisions using Microsoft Word's Track Changes feature. You can diff on the current/previous document revision or between any two earlier versions of the document from the document revision history window.

As much as it pains me, I think I might have to pay these guys...

P4OFC, the Perforce Plug-in for Microsoft Office