Bit of an awkward one, it was said to have been fixed in an update but I have seen it once or twice since:
To Fix simply follow the following steps:
Close Outlook 2003.
Click Start, click Run, type regedit in the Open box, and then click OK.
Expand the registry, and then click to select the following subkey:
Right-click Exchange Extensions, and then click Delete.
Click Yes to confirm the deletion of the Exchange Extensions string value.
Exit Registry Editor and restart Outlook.
Always take copies of your registry before modifying any entries.