I have the following setup:
Windows7 64-bit host - static IP - unique within the LAN
Virtual box Bridged Adapter
Centos 6.5 32-bit guest - static IP - unique within the LAN
When the guest starts it paralizes all the network and the internet
connection becomes quite instable although I can ping any website from each of the machine.
I should also mention that I have forwarded several ports through the router in order for the machine to be accessible via ssh.
Did you ever encountered this problem is it from the guest or from the way the connections for both are set?

Should not happen. Never seen it. Guess you could start off with making the vm client nat. As to what is causing it, you may start with window's performance monitor. Make a log before you start this and see if anything can report what the issue is. Perfmon has thousands of metrics.

Can also look at client next, generally var/ log files. Might use other tools like top.

Hello and welcome to LQ by the way.

Thank you!

Using NAT seems to work as expected without affecting the rest of the local network.
Still I wiil further invetigate it and post the cause if I find it because it looks
strange to me and it is sommeting that should not happen.

You'd have to look around a bit. Sometimes a fix is better than finding out. DNS issue maybe?? Router nat issue?

Thanks for the update.

try to capture network traffic and see if there are any unwanted packets flying aroung causing the network go crazy. On Windows and Linux you could use Wireshark, I usually use tcpdump on Linux instead.

In case you identified any dodgy packets you need to find out where they originate. According to your desxcription I would suspect your VM.
What services/deamons are running? any special software on it?

since you made your VM accessible from outside your netork, could it be it got hacked?

Never seen that before. Did you use a custom MAC address for your VM? Any chance you have a MAC conflict with another system on the network?

Sorry for replying so late. So I changed the VM's MAC and refreshed the interface inside the machine and the problem seems to be solved. I think that the VM took the MAC of the host or something like this. But the ideea is that the problems seems to be gone now. Thank you all the the advice.