Run a command prompt and type the following (hit enter after each line):
C:\
CD "Program Files\Microsoft Office\Office12"
Regsvr32 OWSSUPP.DLL
This will re-register the dll on your system. You should get a windows box popup and it'll normally say that the operation failed, you can ignore this. If this was your problem with Sharepoint/Powerpoint, you should now be able to edit powerpoint files from the Sharepoint links (restart your browser though first though).
Be aware however, that if you install any other Office 12 based applications, you may need to go back and re-do this change.
Paul Leigh