Beating a two session max!

I have a couple programs that use the loopback funtion on defined ports to check to see if there's a copy of the program running. I suspect if they really wanted to do it this way the program could have to statically defined ports (bad Idea in general) and it could check them both to see if there's a copy. If a message is sent it replies.

Concerning:
"(firewall blocking doesnt seem like an option!)" -- Are we speaking of every firewall or the external only firewalls.

Have you tried to block the process from sending out a message? I mean some programs such as ZoneAlarm and other internal firewalls can control the ability for a program to send any messages across any ports.
You could also try to block the port that it's sending information through after it's open.

There are various possibilities of detecting multiple instances, some also includes kernel objects. Using network is not really a smart solution and unlikely to be used.

Also have a look here

hxxp://www.codeproject.com/cpp/avoidmultinstance.asp
hxxp://www.flounder.com/nomultiples.htm

By the way, which application is that ?

Visu

As "Visu" said , there're various possibilities of detecting multiple instances . One of these coul be a semaphor declaration . To avoid runing of another instances , you can declar a semaphor with special name and at the bigining just create your named semaphor and if the API return by ALLREADY_EXIST error , it shows that another instance has been run later .