A user wrote recently:

After installing Office 2000 SP3, starting PowerPoint resulted in the
following error:

R6025 - pure virtual function call

Selectively disabling add-ins revealed that the SharePoint Portal Server client PowerPoint add-in was causing the problem
(PKMOfficeAddIn.PwrPntUI.1). I disabled the add-in by setting LoadBehavior to zero in the following registry key:

HKLM\SOFTWARE\Microsoft\Office\PowerPoint\Addins\PKMOfficeAddIn.PwrPntUI.1

See also, http://support.microsoft.com/?kbid=817841. Microsoft has now identified this problem and has a couple of work arounds and a "hot fix". You should call Microsoft to obtain the hot fix. The URL contains a link to information about the phone numbers to call.