What is wrong with the internet on one compuer out of a network?

I have an entire home network, which mostly has no problems. I recently installed a new copy of Win XP on one of the systems and now the internet on it is unbareably slow. I can't say it worked perfect before the format, and the problem is only on that one system. Thinking it might have been the NIC, I also installed a USB Wireless card, but it hasn't sped anything up. There's still some driver issues, but it doesn't appear to be anything with the ethernet drivers, as those have been updated. I'm not a computer dummy and am quite familiar with most common issues, but this has me stumped.