Today Microsoft released service pack 2 for Office
2007. You can get it from here service pack or use
Windows Update. From my point of view, very important are modifications done with
Outlook.

Microsoft Office Outlook

• Performance improvements that apply to the following general responsiveness
  areas:
  
  • Startup
    Removes lengthy operations from initial startup.
    
  • Shutdown
    Makes Outlook exit predictably despite pending activities.
    
  • Folder View and Switch
    Improves view rendering and folder
    switching.
• Calendar improvements
  Improves underlying data structures and the general
  reliability of calendar updates.
  
• Data file checks
  Greatly reduces the number of scenarios in which you
  receive the following error message when you start Outlook:
  The data file 'file name' was not closed properly. This file is
  being checked for problems.