Slow network

Well, could you tell us how many machines are connected through the one?
couse bridging multiple connections through one machine can be a pain in the butt, as all computer are sucking up your Bandwidth
so actually you shouldnt expect anything more than slow connection.
i dont know if there are any 'routing' software out there to increase/decrease each computers max bandwidth according to the priority, havent really studied the subject..
but maby i can help with google, moment...............OH! it just hit me, your problem is this! -> http://www.speedguide.net/read_articles.php?id=1497
Windows XP has A TCP limit on '10' connection, to prevent your computer from turning into a zombie or something, so try that one, and if it doesnt help, you can read around on google on 'ICS' (internet connection sharing) and you will probably fint tweaks,tips and answers

blimey the tcp thing didnt hit me at once, i must be getting old ;)

 

Nope.. But you can implement a type of quality of service and ensure that a minimum amount of Bandwidth is reserved for either a application, or a system. The generic (and actually not so correct term) for this is QoS.

ermm TCP handshake attempts / sec and TCP connections are not the same thing...

I would first start with the system itself and see if there is anything wrong with it, then look at the network level of things.
--> What is different about the PC that is connected to the internet compared to the others? Do you have any other applications that are resource heavy on it that you dont else where?

By the way, I hope you have a router/modem on the outside of your PC that shares its connection with the others, and it isnt visible on the internet. If it is, you are asking for trouble, and perhaps have some malware on it.

Some tips:

From CMD prompt on xp do a "netstat" if you have millions of stuff here you could have had your system compromised.