When launching Outlook 2007 or 2010 you receive an error "Cannot open default email folder". This seems to happen somewhat more frequently in Windows XP than Windows Vista or 7. After the error, Outlook simply closes.
The concept to the fix basically is to rename the .ost (offline folder library files for Outlook that make up your email folders) files to .old so they are rebuilt but you have a backup incase the worst happens (otherwise you could just delete them). To find these files, make sure you have your folder options set to show hidden files. Then, navigate to c:\Documents and Settings\<user>\Local Settings\Application Data\Microsoft\Outlook (windows XP) or c:\users\<user>\AppData\Local\Microsoft\Outlook (windows 7). Here is where you will find one or many .ost files. Rename them all and change their .ost part to .old and then relaunch Outlook. Outlook will load and rebuild all the folders.