Limited Internet Connectivity

Hi All,
I have a desktop PC connected via ethernet to a router / modem which is the hub of our home network.
Other PC's and devices connected wirelessly to the network have no problems with connecting to the internet. However, the desktop PC appears to be connected to the internet, but connection seems to be extremely limited and slow or non exixtant. The PC is a HP Pentium 4 running XP SP3. I have tried changing ethernet cables and switching ports in the router and switching off firewall with no success.
Could someone pease give me some guidance as to how to troubleshoot this problem? :(

thanks