I think vista always redirects applications to and from the virtualstore directory even if mirc was just moved there and was previously installed in program files on xp or whichever. So it would basically ignore whatever is in the install directory (program files\mirc) and redirect mirc to virtualstore which would make it like a new/clean installation.

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I think I'm going to install vista on vm again and learn all I can about this because I get a lot of questions about it myself and I haven't yet used vista (for everyday use) because I don't like it.