Configure Windows Server As Both ISA Server And AD DC?

I know the best thing one should do is install an ISA server and an Active Directory Domain Controller on separate servers. But for some reason, I want to configure my Windows Server 2003 as an ISA server as well as an Active Directory Domain Controller. What can happen to my server making it run improperly. For example, my ISA server will function mainly as a Firewall, I wonder if all the rules I create in ISA server determine what I mean it should work or I have to consider any affection from Active Directory DC to those rules?I have to ask this question because I've already installed both ISA and AD DC on my Windows Server 2003, already created an Allow. All rule (just for testing connection) in ISA server but I can't ping from all the clients to my server, all the clients can ping each other and the server itself can ping to all clients. I just think that may be the affection from AD DC to my AllowAll rule in ISA server.

I successfully created some rules that worked greatly as they should work but when I hadn't installed AD DC yet (just installed ISA), however right after installing AD DC, it still worked well (this made me think that AD DC didn't affect to how ISA works). Then I configured a little related to DNS server, and maybe something I don't remember exactly and now I can't ping from my client PC to my server. My network is virtualized using VMware 8 workstation and GNS3, at first there is no need of GNS3 because I just want to test my internal network (some clients connected to the internal interface of ISA server). Depending on VMware that I can't be sure if it is some fault of VMware or any other thing but my configuration on the servers.