Reading Layout:
Select Tools > Options > General tab and turn off "Allow starting in Reading
Layout".

Markup (if you use Word 2003):
If Tools > Options > Security tab > "Make hidden markup visible when opening
or saving" is turned on, documents that contain tracked changes will open
showing markup (i.e. with track changes turned on).

Note that if you turn off this setting, you may distribute documents that
contain tracked changes without being aware of it.

From the Word online help:
"Make hidden markup visible when opening or saving" displays all comments,
annotations, deletions, and other types of revisions. If you use the use the
Show menu on the Reviewing toolbar to hide some or all of your revisions, and
you select this option, your revisions will appear when you or another user
opens the file. This option does not affect text formatted as hidden.