There are always lots of tasks running in the background but before we look at those, does this problem occur when you are looking at one website in particular? I had exactly the same thing happen two years ago when I was hooked on Big Brother and kept visiting their official website. 100% CPU usage and the computer would grind to a halt. I suspect that it was something to do with Flash. As soon as I came out of that website the CPU would start to recover.
If it happens with all websites, then try
x-rayPC which will look for and identify malicious programs running in the registry