In order to make DCOM applications work in Windows Server 2008 you need to do two things. Installing the Application role is not needed (but it does add the COM+ network access as in 1) below).
- Allow "COM+ network access" in firewall rules (type firewall.cpl from a command prompt and on the left you'll see the setting). COM+ is somewhat of a misnomer since it is also for DCOM. COM+ Network Access is a preconfigured program in the exception list that you can just check off.
- Add your DCOM server to the list of program exceptions in the firewall.
A quick test of DCOM access is the DCOM test client/server app.