Are you sure mIRC didn't just disappear into your tray?
There have been no reports of a clean copy of mIRC ever quitting randomly without any error messages, so there's little to be suggested- you're treading in unchartered waters, and unfortunately we have little information regarding your system. Programs don't just close unless there was a problem with the machine. You will probably have to do a little more investigation and find a way to reproduce this issue reliably.
- argv on EFnet #mIRC
- "Life is a pointer to an integer without a cast"