Internet connection sending mass data

I have a client with a home computer. Due to problems he was having with a new printer he purchased, he upgraded his Windows ME to Windows XP per the suggestion of a sales person. He must have done alright being pretty computer illiterate. His computer seems to work fine and everything seems to be in place and his printer does not have the problems it had on Windows ME. However, it takes FOREVER, like 10 minutes, to load a web page, if it doesn't time out. After poking around, I noticed that his modem connection status shows a constant increase of sent data. He uses a dial-up connection to att.net. For about 40 minutes, we were connected at 49,200, and in that time, more than 22 MB had been sent over the modem. I finally disconnected. Some sites time out including windows update. All I tried to load was yahoo.com and att.net. Microsoft.com even times out. What is it sending??? How can I get it to stop, so he can have his browser functionality back? I thought of a virus, but his Norton Antivirus 2001 that he had on Windows ME is not compatible with XP, so he doesn't have virus protection at the moment. All he does is check email and rarely even uses an IE browser. He doesn't really know how to do much else. How can I troubleshoot this and prevent this mystery transfer of data from his computer when connected to the internet? He didn't have this problem on ME.