Did you uninstall the trial version Office?
Verify if you face this issue in Outlook 2007 or Outlook Express?
Office Ultimate 2007 retail can be installed only on two computers, only Office Home and Student 2007 retail allows activation on three computers which doesn"t include Outlook 2007.
If the issue is related to Outlook 2007, perform a complete install for Office 2007 using Add / Remove Programs in Control Panel.
1. Select Microsoft Office 2007, click on Change > Add / Remove Features
2. Select "Run all from My Computer" from the dropdown for Microsoft Office and continue the installation