When I boot my PC, a shrill sound continues from CPU. This sound continues and go up and down in a cyclic order. What is the reason ? What remedial measures can be initiated if this sound is indicative of any malfunctioning.

It is hard to say for sure, but it is possible that it is a fan making that noise.

If you want to diagnose this, and are not afraid of being inside your computer, you can TEMPORARILY prevent the fans from running by unplugging them from the motherboard or sticking something between the blades (do this with the computer off). Do this one fan at a time, and only for as short a time as it takes to see if that stops the noise.

There may be several fans:

• Case Fan
  This will be the easiest to find and debug. It should be on the back of the case somewhere.
• Power Supply Fan
  This will be INSIDE the power supply box, and thus hard to get to. Do this one last, as you'll probably have to stick something (PLASTIC) through the vent holes to keep the fan from turning. Use something plastic, but strong, really.
• Processor Fan
  This will be a small fan mounted on the motherboard, with hopefully a small connector nearby.
• Video Card Fan
  If you have a fancy video card, there might be a fan on it. It will be on an option card, and similar to the processor fan.

Remember, only keep a fan stopped for the smallest amount of time it takes to determine if this was the source of the noise. Then reconnect it and go to the next fan--powering off in between each test. Be careful! I don't want to fry your computer, but my advice comes with no warranty.

Once you know which fan it is, you can Google around to find a source. CompUSA used to be a good place to get these, but they don't have brick & mortar stores any longer.

Good Luck,
WallyH