In Windows Vista and 7, miRC has to store its data on that path due to read/write permission. Since Vista/7 doesn't allow to write in program file in many cases (after you installed the program).
Of course you can change that permission roles. But my suggestion is to move contents of both mIRC directory in Program file & the mIRC directory in AppData\Roaming to a single folder in another drives or to root folder of the Windows installed drive. (Which is mostly C:\)
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