I hope this might help a few people. I have been reading some of the other posts on mirc crashing in windows XP. One of the suggested fixes replaces /close with /window -c - this got me thinking. I run a channel where several people use an fserver script which can be heavy on the hash table usage. When using mirc 6.12 with windows XP - I was able to track logs and found that when users typed "CLOSE" "EXIT" or "BYE" in one of the fservers, mirc's default closing action would intermittently cause mirc to crash. I had several users start logging - and in each case, the last thing on the logs would be an fserver close command being typed.

By stopping mirc's default closing and using a /window -c command, the users have reported that mirc has stopped crashing. Here is the code I used, if anyone wants it.
on *:serv:*: {
if ($1 isin bye exit quit) {
/window -c =$nick
halt
}
}

Good luck folks,
Kreigan