Check the temperature of the pc, use the bios to see the correct temp'. It should be in the Pc Health choice box. What cpu and mobo are you running. Also check the connections at the back of your pc.
Are you using explorer 6 or Netscape? Could be a conflict. Go into My computer - right click and go to prperties. Check the device manager and see if there is a driver update or any conflicts.
Also try Control Panel - system - advanced tab - CLick on the Performance "settings" button and click the choice to adjust for best performance. It will make everything go grey again like win2000, but it will make your pc run alot better. If you like the blue background turn it on individually with the "use visual styles on windows and buttons."
Also check your cables etc are not blocking air flow inside your case.