Dat port forwarding

[RaZ]

We have 2 PC here but only one connected directly to the internet.
The PC 2 is getting net through a crossover cable from PC 1.
Filesharing is enabled and we use it near daily, net working on both of the PCs.

PC1: me
PC2: my brother
We don't have router/swtich/hub or anything like these, only a modem and it is connected to PC1.

The problem:
Anytime when I want to play a game on PC1 I need to forward ports, if I don't do it then every game tells me "Connection lost" (in case: MOH 2010, BF BC2), while loading or even "Connecting to online services" (in case: MW3) or "Could not join server (1)" (in case: BF3).
If I open the small tool (Smart Port Forwarding) and I forward ports I can play.
Meanwhile ff PC2 starting to play any game my bro doesn't needs to do anything, just play.

I have reinstalled my pc ~2 weeks ago and before the reinstall I could play without any port-forwarding.

We could play together before, but now we can't unless I use the tool.
I have tried everything: disabled win7 firewall, disabled (and removed) NoD32 (I have moved to Norton for hope), I have tried without any antivirus too.

Does anyone have any idea why do I need to do this?

[Nekochan]

Try to create a Virtual Server and bind one local IP to "server" pc.

[RaZ]

Try to create a Virtual Server and bind one local IP to "server" pc.

Can you help me with that?
Search over the net doesn't say anything for me about this, I have never used Virtual Servers.