Unwanted default browser changing problem

[Mark Taintor]

My wife, bless her heart, is having a hard time changing over to OS X 
completely. As such, she continues to use Opera 5.0 run in Classic. The 
problem is that whenever she opens Opera, it changes the computer's 
default browser to Opera. All that she has to do is open Opera and the 
default browser changes to Opera. No going through System Preferences. 
It just changes. I can reset the default browser in the System Prefs, 
but as soon as Opera is opened the Pref changes. This doesn't happen 
when any other browser is opened, Classic or otherwise.
Any thoughts on what I might do to stop this from happening?
Thanks,
Mark

Mark Taintor
Chanhassen, MN USA