I looked into the issue fairly extensively the first time around, I'll try again but without being able to reproduce it I don't think I'll fair much better.
I have just performed tests under five different Windows installations that use XP SP2 and Vista and tested mIRC using a debugger/memory checker to catch memory leaks and was unable to reproduce or detect any issues.
mIRC itself contains no memory allocation routines that allocate non-paged pool memory, so my guess is that the issue is external to mIRC. Since this issue only seems to happen on some installs of XP and not others, it is most likely a driver issue.
I can't think of a way of pinpointing the actual cause without being able to reproduce it unfortunately.
