From all what I have read and interpreted, xDSL/Cable+ modems are Routers just not in sense we are use to.
For sending and receiving files, a problem doesn’t necessarily mean is on your end.
In Regards to receiving files,
Is this over DCC server? Or is this done over regular means (/DCC send Antares)
By regular means,
Are you using third-party software firewall? If so what is the name of the software firewall? Is it a rule-based software firewall and you using stealthed rule-set or paranoid rule-set? Paranoid rule-sets restrict communication to specific services like surfing, checking/sending e-mail, connecting to IRC servers. If this is the setup you using you will need to configure rule to allow TCP initiating connections from local PC to remote machines, by default temp range (1024-5000).
Now the other user who sends file send request needs to have everything on his/her end to receive TCP initiating packets from remote machine (and in this case it would be from you), so if user has Router, forwarding port or setting him/her self on DMZ is the first step. If user has software firewall running, it needs to be configured also….
If you have any questions, don’t hesitate to poster further.