Angus,

Your outlook may not be set as your default mail application. Try performing this command switch for Outlook 2007 to enable it as your default mail client and try again.

To run Outlook with a command line switch, use the following steps:

Windows XP:

• Click Start.
• Click Run.
• Enter the following in the Run dialog box (with quotations):
  "c:\program files\microsoft office\office12\Outlook.exe" /checkclient
• Click OK.

/checkclient - Prompts to make Outlook the default application for email, news, and contacts.