Windows installer runs when opening Office 2003

I have both Office 2003 and Office 2007 installed on my XP Pro machine.
Office 2007 is working just fine, but in the past month or so I see the
Windows Installer preparing to install message every time I start Word,
Excel, Access, or Publishing 2003. All four programs open after a minute
or so; in Word I get an error message: "An error occurred and this
feature is no longer functioning properly. Please run setup and select
Repair." The other three programs do not display such a message. I have
not installed Powerpoint 2003.

I have tried running Setup from my Office 2003 CD, but I get another
error message: "Patch package could not be opened."

I tried running "Detect and Repair" from the Help menu in Excel 2003 and
got the same patch package error message.

Is there an easy solution to the problem or is it time to give up on the
earlier version? I use Office 2007 much more often than 2003.

I have downloaded the Windows Installed CleanUp utility as suggested in
another thread, but I'm not sure whether running that will help.

Bill