This is a concern for many developers, not just the ones that use Opus.
The initial write into Program Files goes ok, BUT subsequent writes can be redirected into a "Virtual Store" to "protect" the original install -- that's where things can start to go haywire with things like updates.
With the advent of Vista, I have stopped putting anything that can change while the pub is running into the Program Files directory.
Microsoft has stated that the Program Files directory is not the right place to store data.
I now put user specific data into a subdirectory of the current user directory. See attached pub -- it writes a text file to a subdirectory in the user's My Documents directory.
Some devs are avoiding the Program Files directory altogether and installing everything into the user directory, it is an easier option but against guidelines.
The updates feature I have decided not to use because of these issues.
Here's another good reference to this issue:
http://www.sonsothunder.com/devres/revo ... env007.htm
Paul