I have noticed that if a widget's preferences get corrupted, that widget's CPU usage can scale, and use all or most of your CPU cycles. The fix is to use Activity Viewer to list processes by CPU %, locate the troubling widget, and delete that widget's preferences file. Upon re-launching the widget, all should be well.
I didn't notice I had this problem until performing some intense processor activites. So if you feel your computer is a bit laggy, check out your widgets' CPU usage.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051017143257926