Yeah, gotta same problem as well as yours. I have a computer in my school lab and another one in the classroom behaves entirely like this:
No processor activity or HDD activity, but still not responding even when I press Ctrl+Alt+Del... Sometimes connecting to the net aggravates the situation more. Starting the programs when the computer is connected to the net is like heck. The mouse pointer with the hourglass keeps on showing, but no signs of processing in computer... It's like uh... unresponsive... After a few minutes then the whole programs I've clicked splashed in front of my monitor screen... Tried defragging the pc and scanned for viruses, but still the same. Only the operating system started 'gunking' up after a month of installing it.
Well the nearest solution is to reformat the whole comp and reinstall Windows XP. I may be wrong, and please correct me if you have a better solution than this.