A few things to consider:
i) Which ISP are you with? I know a colleague has had web performance issues with TalkTalk until he changed his MTU setting on each PC/laptop.
ii) Is there a 'cache' set in internet explorer ? If it is a fresh XP install (unless it is some manufacturer specific disk) then I can't imagine so but worth asking.
iii) If you do ipconfig /all from a command prompt on both are you seeing same DNS servers listed? I'm guessing DNS is not an issue as you are saying that pinging internet sites (presumably by name) is fast.
Just some ideas.