I noticed a few releases back, that mIRC stores all of it's files (except the mirc.exe) in the logged in users app data\roaming profile. Why did this change from storing everything on the local C: drive? Thanks

This change was made at least ten years ago :-) Modern versions of Windows do not allow applications to modify files in the "Program Files" folder. This means that mIRC, like all Windows applications, has to save settings in the user's AppData folder. If you want to use mIRC as a portable application, you can install it in your Documents folder using the "portable" option in the installer.