Some users report receiving this error:
Failed to open message store.Microsoft Office Outlook Offline Folders  : The file C:\Documents and Settings\username\Local Settings\Application Data\Microsoft\Outlook\outlook.ost is in use and could not be accessed. Close any application that is using this file, and then try again. (Error: 0)
This error affects Outlook users who link to a SharePoint calendar to Outlook. If you remove the calendar from your profile, you will eliminate the error.