Does this happen with every network you try to connect to?
Try
/server irc.dal.net 6667,
/server irc.undernet.org 6667 and
/server irc.efnet.net 6667. Make sure you're not using a strange port to connect.
If you can successfully connect to those networks then I guess it's a server-side issue and you will need to contact the network administration for advice.
It could be an external program causing the issue. As
Section 6 of the mIRC FAQ (written by Krejt) suggests:
"NetCruiser, Twinsock, and Internet in a Box are not 100% Winsock compliant, and some users experience DCC problems with mIRC and other programs... complain to Netcom or the others... Some people also get the error "unable to create socket" on IBOX and other winsocks. If this happens, try to connect again (choose File/Connect or type '/server'). This sometimes worksRegards,