Vista virtualization makes for "weird" outcomes when a program tries to open a file with write access when the file has no write permissions in folders such as program files.
Most likely you are actually accessing a file in the virtualstore instead of the one in program files.

Really 2 things come to mind with that:
-this "feature" of mirc supporting settings if they exist in program files, should have never existed
-more importantly Vista virtualization should have never existed(and should be turned off). Results vary wildly.