so running just that clean mIRC (2f63a83968f9586fe4fb48134253619c) it still freezes up for you? are you not running the UPP install at the same time? what other activities are you performing while using the program? how many networks and channels are you on? any DCC's active?
I'm also running mIRC under win7 64bit ultimate and experience no problems what so ever (having NOD32 as a virus scanner, with real-time file system protection turned on). However, I'm not connected to any SSL enabled networks at this time.