I was having similar problems and may have just found the answer just minutes ago. According to Microsoft, this may be a problem with the phishing filter. If your web page contains alot of frames or if you browse several frames in a short time, it causes high CPU usage. There is a resolution available from Microsoft. You can download the 928089 package or you can disable the phishing filter. You can find the download by searching for KB928089 at Microsoft.com.
I'm having an internet related problem. If my internet idles for somewhere around 1-3 minutes and more, it gets really slow and internet explorer says that it cannot display the page. I then click refresh and it instantly loads with no problem. Sometimes, the page does manage to load, and it's good from there. I am using Internet Explorer version 7, but I don't think it has anything to do with this because I've been using this version for months and this annoyance started several days ago. Also, I discovered that me constantly using the internet connection does not affect the lag that comes. I get the big lag somewhere between 1-3 minutes regardless of what I do. What should I do?
I'm running Windows Xp Home SP2 on a 2.4 Ghz P4 with 512 MB of RAM.