As L3thal pointed out, this has been posted about before, so you can try out the Forum Search
feature to find the other threads.
As far as I know there is not a fix for this, and I also think it only happens with XP/2K systems. Try disabling any firewalls/routers that you have and see if it helps, aswell as making sure you have all available Windows updates
Oh, and mIRC is a chat program - there is next to no reason you should be using it to download from bots in lots of channels. Khaled (mIRC's author) disapproves of using it for this as do many mIRC users in general. If you use DCC normally and practically (and perhaps legally?) then it shouldn't cause you too much hassle - not to say this is an excuse, but I don't think it's directly an mIRC issue anyway. It doesn't happen with me and I'm on XP Professional, mIRC 6.14, nor does it happen to that many people in general.
Either way, I hope that in the future you come up with the solution - if anyone does ever find out how to prevent it, it'd be most appreciated if you'd post here so we can direct people to it!