Recently after migrating to Windows 7, I’ve been encoutering the this message a lot from Outlook 2003.

After poking around a bit (like removing and re-adding my mail account, removing and re-adding my personal and archive folders, etc), the only solution that worked for me is to force Outlook to recreate the OST file. For those who are interested the location of the file is at:

Windows 7:
%userprofile%\AppData\Local\Microsoft\Outlook\

Windows XP:
%userprofile%\Local Settings\Application Data\Microsoft\Outlook\

Just rename all the files with the .ost extension to something else and try to start Outlook again.