Check the problems listed on
this page first.
Double check the setup on your router is correct and that the DCC port settings in
ALT+O > Connect > Options > [Advanced] have been set correctly. There's also a good chance it's the sender's end, try getting from a few sources.
Sadly, there have been lots & lots of posts related to downloading in mIRC and problems with it. You may like to use the Forum
Search feature for the word "download" and expand to 'All Forums' and 'All Posts' for best results.
Remember, file trading on IRC is not what mIRC or IRC is meant for, it is meant for
chatting and getting a few files from friends etc, not mass file trading and a lot of it is illegal (if you're downloading mp3s, movies or software etc). It is not suprising people have issues with FServes etc when mIRC is not properly designed to handle such great loads (e.g. 3GB files etc). Please read
this post.
Regards,