Support for this was first added in a post Service Pack 1 Hotfix for Outlook 2007 and later in Service Pack 2 as well so I suggest that you first check that Outlook is fully up-to-date before attempting this fix. Outlook 2010 also offers support for this.
ExtractOrganizedMeetings Registry Key
Once updated, you’ll have to add the following Registry value;
When this key is missing or set to
0, then you cannot accept meetings that have been sent to you in a 3rd party/unknown calendar format.
When this key is set to
1, then an attempt is made to interpret the 3rd party/unknown calendar format so you can accept it.