(WinXP) Buttons in browsers (IE, FF) non-responsive

This is a really strange one. A friend's PC (Running XP) has suddenly developed a problem where links and buttons on web pages become unresponsive. The problem occurs with FF 3.5 and IE8. Sometimes a link will become "live" if you do a toggle (ALT-TAB) to another program and then toggle back to the browser. Other times, the toggle doesn't fix the problem. I know this isn't a very good explanation, but I'm having trouble even explaining what's happening. Anyone have any ideas where to start? Thanks -