The XP SP3, Vista and 7 firewall is really simple to configure for DC++. Launch DC++ and you will be prompted with the following:
Click on Unblock
That's it! DC++ should now work with the firewall.
If you did not see the popup box shown above, check the firewall settings. In Windows XP Service Pack 2 & 3 or Windows Vista: go to Start Menu -> Control Panel -> Windows Firewall -> Exceptions Tab. For Windows 7: go to Start Menu -> Control Panel -> Windows Firewall -> Allow a program or feature through Windows Firewall
The firewall should be On
If DC++ is not in the list of programs, click on Add Program. Find where dcplusplus.exe resides on your computer (typically c:\program files\dc++\). Click on OK.