1. Try running System REstore
2. Try deleting, and then recreating, your email account(s). Go to Tools > Accounts > Mail. Click on an account to highlight it. Click 'Remove'. Then click 'Add' and recreate the account. (NB: Don't attempt this unless you know the relevant settings for your account. These are available on the website on the website of your ISP or email provider. Of course, you'll also need to know your password).
For further suggestions, see here:
http://support.microsoft.com/kb/813514/
Chris