Wifi or wired? Works on one but not the other?
Network drivers fully up to date?
Any difference in antivirus/malware software on working/failing machine?
If you plot these performance counters in perfmon:
\Network Adapter(x)\Output Queue Length
\Network Adapter(x)\Packets Outbound Discarded
\Network Adapter(x)\Packets Outbound Errors
where x is the network adapter you are using, do you see anything unusual? Perfmon instructions at
http://technet.microsoft.com/en-us/l.../cc749115.aspx