Log Files Written to Desktop

A client of mine has an interesting situation – log files are being written to the desktop.
ALWAYS, when running Outlook Express
SOMETIMES, when running Excel

These are the files:
cleanup.log
offline dbxi
nbox.dbx
folders.dbx

What are these files and how to avoid creating them? I can’t find information in Excel, Outlook, Outlook Express,or Windows

Thanks very much.
Helen