dont know about reciveing files but if your getting it during sending try this

in mirc press alt-o
connect/options/advanced set dcc first and last ports to say 6001 to 6010

When u send a file mirc tells the reciever what port to come on in on, by default its 1024 to 5000 starting at 1024, this can been seen as a hack attempt by some defence programs and they shut it on down, setting the ports to some random value between 5,000 to 10,000 normally works.