If you have been using Vista, Outlook 2007 combination and using POP3 (Non Exchange Server) for downloading emails – you may be experiencing slow email download. This might be due to the auto tuning of TCP/IP in Vista. Here is a Microsoft Support KB that helps to resolve this.
Update (20 April 2007): Similarly if you feel that search is slow with Outlook 2007 (I liked Lookout, unfortunately it doesn’t work with Outlook 2007) then download this fix from Microsoft.