I came across this the other day. A perfectly good working machine running Word, Outlook and PowerPoint. PowerPoint then started to fail when closing down. The event log showed the failure to be in MSO.DLL. This was the latest version for Office 2007, all patches applied.
The only thing that changed was the switching over of the Printer from a Lexmark to an HP. I’d made the change and the printer and associated software was all working ok.
The issue was to do with not having a local printer installed. I did delete a local printer entry during the installation of the new printer, the removal of that redundant printer caused the issue.
To remedy, I just installed a local printer using any driver. There is no printer attached locally, but Office does like to have one regardless of if you use it or not.
I’m not saying this will fix all MSO.DLL failures, but if you come across an Office installation that was working fine one day and not now, check the printer setup and see if a local printer may have been removed recently.