Junfeng Zhang, Microsoft blogger states that IE7 might cause CPU spike when you visit websites using AJAX extensively such as Yahoo Mail Beta, Google Reader to name a few. The cause behind this might be anti-fishing and he suggest that in order to work out the problem you need to disable the add-in. Here is an excerpt from his post:

When visiting AJAX extensive web sites (for example, Windows Live Mail Beta, Yahoo Mail Beta, Microsoft Outlook Web Access, Google Reader), IE7 may cause CPU spike. This could be caused by Anti-Phishing.

To workaround the issue, you can disable Anti-Phishing by going to Tools->Internet Options->Advanced, scroll to Phishing Filter, check "Disable Phishing Filter", and restart the browser.

On the other hand Matt states that the problem might not be as simple as it looks as Microsoft has been marketing IE7 with Phishing filter as its selling point and whether disabling it in order to visit the AJAX sites would be a wise thing to do? He suggests that rather than turning off the filter, it would be better to add these websites to a security zone where the filter has been turned off and this would be a better option rather than disabling the Phishing filter.