Q:

Network performance issues on windows 7

One of my machines has started to get serious networking issues. This all started to happen suddenly although I am not aware of any specific changes made to the machine. It is running Windows 7.
Some of the symptoms that was not a problem before:

ssh into the machine rarely works and when it does it might stop after some minute
rdesktop to this machine shows similar symptoms as ssh, if you manage to get in it freezes after a short time, but usually you do not get in at all.
nmap shows: Note: Host seems down. If it is really up, but blocking our ping probes, try -PN. This worked fine before.

However connecting with vnc has no problems at all.
Googling around I have tried some tips, none working so far such as turning off RDC (remote differential compression), turning off firewall, turning off antivirus, turning off network auto tuning.
How can I find out what the problem is ?

A:

Before you say "network" let's understand what LAYER you're talking about here. 
If it's truly a "networking" problem its going to be layer 3 and below.
So... check the following:
Layer 1: Frayed or kinked cable? If you have a managed switch can you see any physical errors on the port utilized by the client? Is the NIC linking at half or full duplex, 10 or 100 or 1000?
Layer 2: Thrown on Wireshark and lookout for any duplicate MAC addresses jibber jabbering back and forth.
Layer 3: Is it receiving a valid address not shared by any other peer on the network?
As for the OS itself... check the usual culprits:

3rd party and OS firewalls fighting each other.
Protocols you're not using yet, like IPv6, uncheck it.
bad DNS configuration
Did you set the connected "network" as Work/Home or Public?

Troubleshoot some more.

