I would give the system a reboot - if you have checked the processes (ctrl+alt+del), and there is no CPU usage going OTT or unwanted program running, disk trying to be read in the dvd drive, no virus software running (which you would notice in process anyway). Plus! What ever your running on the pc at the time, I.e. game causing it to go crazy. So Yeh, give the pc a restart.
If that don't work, pass it on too the experts