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I am new Virtual Box. I have a host that is Win2003 and I created a guest that is Win2003 server also. I am using bridged networking. Everything seems fine. I can only see certain ports open on the guest such as ftp and 3389. I need access to other ports that are open on the VM such as 18100 and 7778. Should Bridged Networking not let this happen?

Bridged networking works at a lower level than that. "Ports" are a software fiction, nothing really to do with the hardware. Ports are really just an extension of TCP/IP addressing. If the guest has an application monitoring a port, you should get a response from it.

I created a WIN2003 VM. Some of the open ports on the VM are:
80: Server port
3389: Remote access port
18100: Application Server Control Port
I am using bridged networking. When I log into the VM from the host, everything seems to be working as it should. If I create a share on the VM, and can access that share from anywhere on the network. I can also remote desktop to the VM from anywhere on the network. If I run a port scan on the VM from a client on the network while port 3389 is open, 80 and 18100 are NOT. My question is, since I am using Bridged Networking, should I not see ports 80 and 18100 open as they are open on the VM.

Be aware that using Virtualbox does not prevent you from actually configuring your server firewall, security settings and policy, etc.
This is true regardless of any mode you will use for the network.
From what you are saying of your issue and the opened port, it looks like you didn't configure everything fully.

Some guidelines : did you configure the windows firewall properly? Did you enable the default security settings? etc.
For more precise information, you should also have a look at Microsoft Technet.

When you bridge to a NIC you are connected at the lowest Ethernet level. TCP/IP levels are handled by software running in the guest, so that is where you should look if you aren't getting the responses you expect. Have you managed to get two PCs on the same LAN working as you expect?

I'm still trying to figure this out. My guest machine is configured to use bridged networking. It is visible on the network. I have access to shares on the guest, and certain ports such as 3389. I do not have access to other ports such as 18100 which is open on guest VM. I would have thought that since I am using bridged networking I would be able to access the same ports as if I was accessing a host pc. Any idea what I am missing?

You're missing the firewalling. It is somewhere, most likely on your guest. Make sure you disable it or configure it properly.

Oh shoot... I was looking at the firewall on the host and it was the firewall on the guest. I just had to open the ports I needed on the guest firewall.

Thanks.