Sometimes when Outlook is setup to use Cached Exchange Mode, the offline file which is created on your local disk can be corrupted from an improper shutdown of Outlook, among other things. When this file contains errors, it will greatly diminish Outlook’s performance and in some cases prevent it from starting completely.

There are two things you can try.

1. Repair the offline file with a Microsoft utility called SCANOST.exe. I’ve seen this file reside in a number of different locations, mainly within Program Files. It might be best to search for it with Windows or Google Desktop. SCANPST.exe will also work, though you’ll have to go digging for your outlook.ost, which should be near c:\Documents and Settings\ %profile%\ Local Settings\ Application Data\ Microsoft\ Outlook\ outlook.ost. If your file is significant in size, you’ll save a lot of time with the repair operation by not creating a backup. Because if the repair finishes successfully and Outlook still bombs, you’ll need to;
2. Shutdown Outlook completely, including any runaway outlook.exe processes in the task manager. Open Mail in the Control Panel and before you delete your profile completely, write down the server settings! There’s no need to worry about backing up your mail as it’s stored on the Exchange server. (Essentially, you’re deleting that outlook.ost when you drop your mail profile.) Now, start Outlook and it will create a new profile, where you’ll configure the server with the information you wrote down previously.

Outlook will start normally again.