Vista - certificate expiring (self signed)

Hi,

I have a problem with the expiring self signed certificate on our server.
Warning Event IDs 12017 and 12018 occuring every few minutes in the
Application Event viewer.

I have run the 'New-ExchangeCertificate' cmdlet with no parameters and that
has created a new certificate in the same location as the expiring one.

***** However, the warnings continue. Do I need to do something else?? *****

When the certificate actually does expire on August 15th do I need to delete
it? Or ignore it?

There is also a 'StartCom' certificate in the same location, I installed
this so that OWA would work without warnings, which it does provided the
clients have the appropriate trusted root entry.

With this, do I still need the self signed certificates? I guess so as they
are I think primarily for LAN communication with Outlook.

The systems are: -

Windows Server 2003 std 64 bit
Microsoft Exhchange Server 2007
Outlook 2003 SP3

Many thanks in advance for any help or advice with this,

--
Nick Brown
Systechnix Ltd