once in a while Outlook’s data files used to store all your emails, contacts and calendar entries gets corrupted. If you have an Gmail, Yahoo Mail or Hotmail which backs up all your email then you can very easily restore Outlook to default settings. Here is how you can reset Outlook 2003, 2007 or 2010 to factory default settings.
In Windows XP go to "Start" then select the "Control Panel" and then switch to "Classic View" option in left hand column. Then select "Mail"
In Windows7 go to "Start" then select the "Control Panel" and look for "Mail". If you cannot find it type “mail” in the search box on the top right. Then click "Mail".
This will bring up the Mail Setup dialog box with Show Profiles section that we are interested in.
In most cases you will just have one mail profile. If you delete this profile then all your Outlook settings will be reset to default. To delete click Remove and then press OK.