I have had this happen with my Outlook 2007 occasionally, but if I let the process complete then Outlook opens normally the next time. Are you connected to an Exchange server or is this a POP3 mailbox? Are you running in cached mode? If you are using cached mode and this happens every time even if you let the check complete then try deleting the .ost file and letting it rebuild a new one. If this is POP3 you may need to repair the .pst file.