Eugene's observations

Sometimes a PC will hang: no combination of key presses or mouse clicks will cause it to do anything. At such times all that is left to do is to press the button on the front of the PC for more than 4 seconds to cause immediate power loss. But this must be a last resort as it can cause disastrous consequences.

Hard disk drives are similar to an LP record where the read head hovers just above the platter. It is held there by electro-magnetism so that when the power is cut abruptly, before the PC has completely shut itself down, the head looses the ability to hover above the platter and comes into contact with it, this is called a 'head crash'. As most platters spin at 7,200 rpm, this often causes physical impact damage as the disk circuitry has not received a 'park' instruction to move the head to the edge of the disk. Sometimes a groove is ploughed into the surface of the disk.

At 7,200 rpm the edge of the platter is travelling at over 74 miles an hour and as the crashed head drags over the platter surface, the read-write head overheats, making the drive, or at least parts of it, unusable until the heads cool. Often the heat is sufficient to melt the surface substrate locally causing the head to be 'welded' to the platter. It is also common for particles of material scraped free of the drive surface to greatly increase the chances of further damage to the disk.

Apart from these potential hardware consequences, any files that are open on the disk may be cached in volatile memory. If power is lost before the data can be flushed to disk, the file will become corrupt. If the file is critical for Windows operation - such as a registry file, Windows will not be able to start.

Sometimes these problems are repairable, often they are not. So if your PC stalls at the 'Shutting down Windows...' phase of powering off, make certain that there is no more disk LED activity before cutting the power. And if you are able to start Windows after, be sure to run CHKDSK to repair logical errors and ensure backup copies of files are current.