Problem with CDO and Outlook 2000

Hi.
using outlook 98 I use the following to create a MAPI session
set objSession=CreateObject(MAPI.Session)
This works fine.
I've now upgraded to Outlook 2000 and the same code gives me the message
"either there is no default mail client or the current mail client cannot fulfill the messaging request.Please run Microsoft Outlook and set it as the default mail client"
Outlook IS my default mail client.
Then the good old "Activex component cannot create object" message appears.
This is doing my head in.
Anyone got any ideas PLEASE.