Answer 1 Just deleting MSIMIN.EXE probably won't work. You really need to reinstall Outlook Express and probably Internet Explorer.
This shouldn't delete your messages and address book. But to be on the safe side you should back them up. If you can get in to O/E you can use File/Export. If not, use Start/Find to look for *.wab (address book) and copy somewhere safe, then *.dbx for the message files. Back up the whole folder. If reinstall does delete them, putting them back in the orignal message folder should make them reaccessible as long as you do it as a job lot. You can also import from such a copy.
Answer 2. Something has corrupted MSIMIN.EXE but I've no idea what. Could be anything from finger trouble to cosmic rays. I've not heard of a virus doing this but take the usual precautions - update and run your virus checker, update Windows.