Ron Boyd wrote:

Mine started (a couple days ago) the instant I upgraded from Office
2003 (Student version) to Office 2007 (Ultimate) and replaced the new
.PST file with the old one.

Thereby corrupting your mail profile. Never replace one PST with another.
Moreover, when changing versions, it's almost always a good idea to start
with a new mail profile.
--
Brian Tillman [MVP-Outlook]