Connection cannot be maintained

I built an i7 rig for my friend.

i7 920 and a MSI x58 Platinum. We installed Vista Ultimate x64.

We find that the Internet disconnects after boot (almost every time). We have ruled out the router (it cannot maintain a connection to the router, even though the light for that particular Ethernet port is lit up on the router; we tried with another router though and it still couldn't maintain a connection) rather something wrong with the computer, whether it is the mobo or Vista.

When the computer starts, the Internet works VERY briefly before the Network & Sharing Center reports that the Internet is unreachable; I can't even go into 192.168.1.1 (the router). We've had two or three instances when I uninstalled the Ethernet drivers and reinstalled them, and the Internet worked perfectly. A restart later, and it fails again!

Can anyone help with this? Restarting the router helps the Internet for like, 10 seconds, but the Internet then ceases to function.

We are using the Realtek ethernet drivers provided on the CD AND on MSI's website, and I did install the mobo drivers provided... what am I doing wrong? Is the motherboard going to have to be replaced?

The FIRST Ethernet post at least works in some way; the second Ethernet port NEVER works.

This happens both when a static IP is assigned and when a dynamic IP is used for this computer. We'll try using Ubuntu and Windows 7 later today and see if the same thing happens.

any third party firewalls installed on the machine or just using the inbuilt windows one?