I had a similar situation a while back, where I wasn't doing anything other than surfing, and I tracked the problem down to badly written animated ads that were eating all my CPU cycles. Right-click the Taskbar and choose Task Manager, then in the Processes tab make sure that CPU & CPU Time columns are selected (View>Select Columns). Click twice on the CPU column header and that will put the process that is using the most CPU cycles at the top. If it's showing your browser as being the culprit then suspect ads.