Using Application Data has nothing to do with Program Files security settings. Microsoft decided that it makes sense to store settings in a central location under each user. The main reason being that settings for one Windows login do not affect another user's login. This lets different users have different settings in programs without the programs having to implement their own multi-user setup. It's not a bad idea, really. Of course, I don't use that folder with mIRC. I prefer to keep everything in the mIRC install folder.
As for avoiding Program Files, it depends on the OS and your settings. Vista was a royal pain, but 7 is better. For the most part, you should be able to use mIRC in Program Files without any issues in Windows 7 regardless of your UAC settings. The only thing you may have trouble with are scripts that store settings in the mIRC folder instead of the Application Data folder. In any case, I've always recommended to people to install mIRC somewhere other than Program Files. Not just because of the security settings, but because of the space in the path. Not all scripts are set up to handle spaces, so rather than either fixing any script you use or having bugs, if you install somewhere without spaces in the path, you won't ever have to worry about that no matter how your scripts are written. It saves people a lot of hassle that way.
