Incidentally - when your pc is just left on it isn't necessarily 'idle', operating systems have inbuilt maintenance routines and many of the software utilities you might install also install scheduled tasks which may be running like the virus scanner. Most of these are still not a problem when you kill the power but any data that has not been written to the disk can be lost. Thats why defraggers tend to write the data to another area of the disk before they move it back to the cotigious space - it means even if the power is cut the data is 'on disk' so not lost.