msimn.exe is the executable for Microsoft Outlook Express, the default POP3 email application included with Microsoft Windows. This process should not be removed while you have Outlook Express open.
Non-system processes like msimn.exe originate from software you installed on your system. Since most applications store data in your system's registry, it is likely that your registry has suffered fragmentation and accumulated invalid entries which can affect your PC's performance. It is recommended that you check your registry to identify slowdown issues