I have a Belkin wireless setup on one of my PCs and it works fine although somewhat erratic at times. No connection problems however. I suppose it depends which one and what's built in.
I also have Trend Micro and there's a firewall included with that so make sure it's either turned off (more than one firewall=bad idea - the firewall, not the entire program) or you have allowed your P2P programs. I got caught by this one. Drove me mad before I realised what was causing it.
Check for other things like trojan scanners and malware finders. Some have built in stuff that don't like P2P and sharing folders.
Try disabling things one by one (including the Windows Firewall) - you should be safe as long as you remember to turn it back on again as soon as possible but try not to visit too many sites or download anything with them off - and see if it makes a connection.
As a last resort, sign up for a free dial-up ISP and test the thing (in case the programs have become corrupt). At least that tells you the P2P programs are working and the PC firewalls are not the cause .You can then concentrate on things like the router (many do have a built in firewall and some block all P2P and similar programs - and also some ISPs do as well.
Another suggestion (sorry but I do like the less technical stuff first) is to try other P2P programs as well and see if they work at all (uninstall the others first) as, occasionally, they can cause a conflict if common files get overwritten etc.
Very unlikely and shouldn't happen but possible.
Of course, try RobertR's and Lord of the Rings suggestions as they are the most likely. Mine are for "when all else fails................" and learned the hard way over the years (If something can go wrong, it's probably happened to me at some time!).