The following problem has recently cropped up: when the computer has been switched on, the keyboard (optical ps/2) works as normally for a while, yet after about 5 minutes an inputting delay/lag starts to kick in, with letters coming up on the screen 5 seconds after i've typed them. The screen then suddenly blacks out, and the computer seems to go into some kind of standby mode, yet the fan starts making a very loud whirring sound. Once it goes into this mode the only way i can restart is to hold the power button. Could the keyboard be the cause of this, or could there be some underlying problem? Virus scan has come up with nothing. I would really appreciate some help.