It's not mIRC, or if it is, it's a conflict specific to your hardware and/or drivers. Why? Because there are a LOT of people using Windows 7 (both 32bit and 64bit) and mIRC without problems. For that reason, it's a setting on your end either in mIRC or in Windows or in some other program you are running. Or else, as mentioned, it's some hardware or driver issue.
In addition, many suggestions were given to you and you haven't responded with the results of those suggestions (such as the time between freezing that I suggested). You want people to help you and they are... but if you don't respond back with results, no one can do much for you. Troubleshooting is no good without results.