The key phrase here is
it works until a reset of the server then mirc.exe is suddenly deleted.
Surely mIRC would not just delete itself
. And the fact that it happens when you reset the server, ie. when mIRC is not running, would imply that mIRC cannot be the one doing the deleting.
My guess is your server is configured to restore filesystem state on reboot, and that might be related to installations as well