Outloot crashes when PGP destop is active

Hi all...

Firstimer here so be gentle with me

We are using windows XP, Outlook 2010 and the PGP desktop. 

PGP Desktop is active.  The user sets up a meeting.  The invitations are sent. The  attendees reply.

  -  If they reply with no message then Outlook is OK and continues.

  -  If they reply with a message for the owner then outlook locks\freezes and has to be closed using Task Manager.

PGP Desktop in deactivated. Outlook fuctions as normal.

What have I missed...  Can someone help and point me in the right direction to resolve this behaviour ?

Thanks for your time, help and patience.

Regards

Jim