Which OS are you using? First of all, see if Outlook runs in the Safe Mode. You can start Microsoft Outlook in Safe Mode by going to Start>Run and typing in:
outlook /safe
You should see the Safe Mode notification in Outlook's title bar if it starts. If it starts, then backup all your Outlook data. For instructions, refer here:
*support.microsoft.com/?kbid=196492
Use the Office Removal Wizard to remove and reinstall Microsoft Outlook. For more details refer here:
*www.microsoft.com/office/ork/2003/tools/BoxA13.htm
Also, this can also happen if you run into serious problems with third party programs that integrate with Outlook, including anti-virus programs, programs such as SMS messengers, Adobe Acrobat Professional, etc. You may also need to reinstall Internet Explorer to fix the problem, as well as the third party programs that integrate with Outlook.