That looks like Vista to me.... but will give you the options for both XP and Vista
For Windows XP (Service Pack 1 and higher)
# Click Start, Control Panel, double-click Add or Remove Programs
# Click Set Program Access and Defaults
# Click Custom
# Under Choose a default Web browser, select Internet Explorer
# Click OK
For Windows Vista
# Click Start, and click Default Programs
# Click Set program access and computer defaults
# Click Custom, select Internet Explorer and click OK
# Close and re-open Microsoft Outlook
If this doesn't help you then try this....
Download this file. This one fixes the problem for both XP and Vista.