Check for the existence of a running instance of Outlook first.
The following will throw an error if no instance of Outlook is found. If an error is thrown, run ShellExecute to start Outlook and set your Outlook object variable.
On error resume next 'Ignore error if one is thrown
Set objOutlook = GetObject(, "OUTLOOK.APPLICATION")
If ERR.Number <> 0 Then 'No instance of Outlook found
Set objOutlook = 'Insert your ShellExecute code here
End If
on error goto 0 'Reset error handling