First, you need to close Opera. Now find your profile folder. Depending on how you installed Opera, this will either by under your "Documents and Settings" folder or your "Program Files" folder. In my case, it is under "C:\Program Files\Opera 7.60 Beta 4\profile".
Now, open up your Opera6.ini file (notepad is good for this).
The first section of this document should be headed "[User Prefs]". Perfect. Add one line of text under this.
CODE
Title=Your Custom Title Here
.Save the file, then open Opera back up.
That's it. Change it to say "Opera" if you like....or like at least two people are doing over in the Web Browser Discussion forum, change your Title to "Screaming Fat Lady"
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(Do try not be evil and change your Opera-loving friends Opera to say "Use Firefox" or anything, though