Just FYI, any time a program shows "program not responding" in windows, it just means that it's not allowing any interupts to it's process. Often this happens to mIRC because of a script running, and if this is the case you can press Ctrl+Break to halt it's operation. If it's in the middle of a file copy, this might not be a great idea. Just let it go on until mIRC responds again and all should be OK with mIRC and your file. Just reconnect to your network and resume chatting.
Oh, and mIRC is not meant to be a file-trading program.