when power fail suddenly computer switch off so that is it bad for a hard disk or not.it is normally happening 2 days for a week at least but someone say there is no any problem for a hard disk through my question some guys are saying when it's happening continually there might be lot of bad sectors in the hard but I don't have any idea about even bad sectors also.Pls replay me to these questions,
A few times during its life span most likely won't kill it, but the possibility is always there. I don't like the sound of it happening regularly 2 days a week. What the hell is up with your power situation there? Get a good battery backup if it's that bad. Anything that takes electrical power from the power supply has a chance of getting fried during a power spike, brownout or blackout. Protect it while you can.
It shouldn't cause any physical damage to the drive, but it stands a pretty good chance of corrupting the data on the drive (possibly to the extent that windows will no longer boot). I agree that in the conditions described, a battery backup would be a good idea.