Visio 2003 suddenly starts asking for upgrade files

[Jennifer]

I've been running Visio 2003 on this same PC for 5-6 years. (I know, I need a new PC.)

Suddenly (starting yesterday), every time I try to open a Visio document, I get a dialog box from Windows Installer saying that it is "preparing to install". (See first attachment.)

After a few seconds, I get a dialog box from Visio 2003 saying "Please wait while Windows configures Microsoft Office Visio Professional 2003". (See second attachment.)

After a few seconds more, I get another dialog box from Visio 2003 saying "Installation Error: File not found" and asking me for the installation disk. I have a screen shot, but I am only allowed 2 attachments (sigh).

If I click cancel on any of these dialog boxes, the problem goes away and Visio seems to operate normally.

Why am I getting these messages all of a sudden?

It seems to me that this happened a few years ago, too, and they eventually stopped. So far, I cannot find my original installation disk.

[Jennifer]

Here's that other screen shot, not that it really matters.