Strange connection problem

Hi, I'm experiencing a very strange connection problem.

When I download something, it freezes after a while. Same happens to MSN Messenger, when I'm logged in, it just suddenly logs off (losing connection) for like 1 second and logs back in.

I also use Xfire (www.xfire.com for more info), but it crashes not as often as MSN/Internet does.

Also, I use TeamSpeak and games, like Battlefield 2, but those don't lose connection at all.

I'm on a LAN with a switch, and I'm the only computer that has problems with connection. I plugged my network cable in other ports of my switch, with no positive result.

I also tested another computer on this cable, and that doesn't have any problems.

I use onboard LAN, but I also tested another PCI networkcard, and the problem stays.

So the following things are ruled out:
- ISP
- Switch
- Network cable
- Network card

If the above stuff isnt causing the problem, what is?

I use Windows XP with SP2 and all necessary updates. I also have .Net Framework 1.1 installed. I use Internet Explorer. My mainboard is an MSI K8T Neo-FIS2R.

Any ideas of what is causing this problem? Thanks in advance