So you are using the build in internet sharing of winXP? That would explain it..
You should instead use for example winroute, and forward some ports for the dcc, to the ip of the amiga box. The standard dcc send ports are "1024-5000" but the ports can usually be set in the irc clients settings. Afaik, the built in routing of XP does not support port forwarding, not really 100% sure though.
This is a flaw of the winxp routing software, not the amiga or amirc :-)
