As sed above there's no easy way about it.
Get a router or switch and will run far better and can run both at same time :)

yeah mate, modem to switch, then 1 network cable ot each pc

nooo, a router has a broadband modem built in, so you would run 2 cat5 cables (network) to either pc and plug it into the network socket, if your machines are on dhcp it will work with very little setup, if there not then u'll probs need some knowledge of setting stuff up

so would i connect this to my broadband modem then run two wires to the two computers? What ports would the connect to on the PC? USB or Network?

Sorry networking is not my feild so may seem obvious questions.

for a simple one..

http://www.scan.co.uk/Products/ProductInfo.asp?WebProductID=152509

I was hoping to do it by just spending a tenner on a modem off ebay. How much are routers?

why not just buy a router

I currently have my pc running Wanadoo through a normal broadband modem. If i was to buy another modem then set it up to dial into the same account.

Im aware that only one pc would be connected at a time.

Thanks