I'm experiencing the same very annoying problem, also on Windows 7 64-bit (Home Premium, German)
It may very well be that it only happens if mIRC is force-closed by the Windows shutdown process since I usually do not manually close every single application before shutting down. Even so this shouldn't happen in my opinion and I never had this problem on Vista or XP.
I'll try to manually close mIRC for a while and see if that helps, but it is not all too easy to find out since it happens very randomly - sometimes three days in a row - then again there could be two weeks with everything working fine.
