msimn.exe is the file that starts Outlook Express.
One possibility is that there is a shared component between this and Internet Explorer that is causing the difficulty.
To remedy, go to Control Panel > Add/Remove Software, click on Outlook Express, click Remove. You will then be given the option to Repair, which you should take.
Then do the same for Internet Explorer
Another possibility is that you previously had SP1 installed, and have not yet re-installed SP1. Did you do a 'repair' re-install of XP, or a format and install from a clean disk?
Finally, going to Start > Run and typing
with the XP CD in the drive will allow XP to repair / replace any corrupted files that may be causing your difficulties.