Actually, it's Zend Platform that's the problem. I uninstalled all the Zend products that I had on my system and got back to just XAMPP. At that point, everything worked fine.
The first Zend product that I re-installed was Platform 2.2.3 and the problems came back. I un-installed it again and everything went back to normal.
<edit> I forgot to mention that I followed Marcel's PDF on installing Zend Platform on XAMPP. It doesn't work quite as described, but close enough. I also un-commented the Zend Optimizer line in the PHP.ini file </edit>
I then installed Zend Studio(5.5.0), Optimizer(3.2.0) and Guard(4.0.1) and tested after each install. Everything worked fine.
I placed a ticket to Zend regarding this and the response was that Platform doesn't work with XAMPP 1.5.5 because of PHP 5.2 and that I should downgrade my XAMPP.
I have a better solution. I won't worry about installing Platform until it catches up. I was just testing Platform to see how it worked and if there was any advantage to using it.
I have sites written with PHP 5.2 that I need to be able to maintain and I really don't feel like re-writing them in 5.1 or 4.x just so I can test a piece of software. Unfortunately, I don't have a testing computer to play with this so I was working with a production machine. Maybe this will give me an excuse to buy a test system.