The other questions need answering too, but given a completely clean install I'd say there's a chance windows shutdown improperly and/or you close mirc by shutting down windows. Generally windows gives programs time to respond to a shutdown request but if they don't react in time Windows kills the process, which could cause data loss. Make sure you close mIRC before shutting down windows, if that's how you're doing it.

- argv[0] on EFnet #mIRC
- "Life is a pointer to an integer without a cast"