From what your saying, it sounds like you have some scripts loaded (though you havent modified them since 6.17). If thats the case, try using a fresh copy of mirc with no scripts loaded and see if the problem resolves itself. If it still happens, try posting some more detailed information regarding the 2 PC's (what operating system, CPU/RAM, service packs, etc etc) so we can either 1) help confirm these as possible bugs, or 2) help you track down whats causing it on your PC. You'd be surprised how many issues/bugs seem to disappear once a clean installation is used. Sometimes small changes in the scripting language, or how mIRC processes the scripts can affect your scripts even if you havent touched them.
