There have been numerous reports about this happening, and I think almost all of them have occured on Windows XP and 2K. I have not seen a specific fix for it yet. I think it happens most often with 6.12 (or any of them in the 6.1x range). I also believe it only happens when you receive really big files.
This is however, not a constant problem for people and doesn't usually occur with multiple versions of mIRC and multiple OS's, so perhaps it is your computer some how. Perhaps you should try the manufacturer's website or the Microsoft User Groups
To be honest, if you're using mIRC for it's actual purpose, you should not be DCC receiving very often. Make sure you read this post
I suggest you also use the Search
feature and search for something like "Freeze" and/or "DCC". Expand to 'All Forums' and expand the date range to 'All Posts' for best results.