See:
http://www.mirc.co.uk/ircintro.html
Also Popular IRC clients (in no particular order) : mIRC, Virc and Pirch. You can get an IRC client by anonymous ftp from several sites (use the one closest to you)
FTP locations
ftp://ftp.undernet.org/pub/irc/clients/windows/
ftp://cs-ftp.bu.edu/irc/clients/pc/windows/
ftp://papa.indstate.edu/winsock-l/winirc/
You can also visit the programs' World Wide Web pages to download the clients;
mIRC http://www.mirc.co.uk/index.html
PIRCH98 http://www.pirchat.com/
Visual IRC '97 (Virc'97) http://www.megalith.co.uk/virc/
OrbitIRC http://www.orbitirc.com/
TurboIRC http://members.xoom.com/TurboIRC/
LeafChat http://www.leafdigital.com/Software/leafChat/
Fahrenheit Carrera http://www.sofasitters.com/
IRC Gold http://users.erols.com/smerchant/index.html
XiRCON http://www.xircon.com/
Internet Tele Cafe http://www.telecafe.com/telecafe/
Masquerade http://www.hijinx.com.au/
Hope this helps!