If it works on others comp's but not his, then its a program thats most likely blocking it. Since you've tried disabling the firewall, it could be an anti-virus/firewall program such as norton or mcafee, where it sets program allowances to the internet.... You should try disabling all other defense on the computer, and see if it works.
If it works with all your defense off, then start turning them on one by one to find out which program is blocking DP. When you find out which one, you can allow DP in the program settings in the anti-virus/firewall.