ICS sucks u should get your self a router and put it in between the lan and the cable modem, in fact moust routers have multiple ports for lan so are actually lan hub/switches as well, so you kill 2 birds with one stone.
Anyway, go into mirc and click tool/options/connect/options/advanced, then set your mirc dcc ports to something a bit more realistic than 1024-5000, like 1050-1059 (example only i dont knwo what else might want them, i uses 8000+ myself), then goto PC1 and service (forward) them ports to pc2, and if u can be shagged service port 59 and/or 6060 as well. and if u do 59 or 6060 in mirc click tools/options/dcc/server enable it set the port and if u want chat send and or fserver listening (dont do fserver inless u have one eh?)
And lastely id like to mention ICS sucks, oh did i say that before? well it does.