Depends on when it happens. If write caching is enabled and the machine loses power before the cache is flushed to the disk, whatever file it was writing will often be corrupted. These little errors tend to accumulate until you have all sorts of weird unreliable behavior. Even proper shutdowns don't seem to prevent Windows from eventually deteriorating.
There's a lot of information here that was certainly very true in the dark ages of computing (say, pre-1990), but modern hard disks (anything more modern than MFM) are not sensitive to sudden power loss. Modern operating systems (MS-DOS 3 and later, roughly) do not instruct the disk to park the heads at shutdown which implies that the disks do reliably protect themselves else there would be the risk of a failure every time the PC was turned off.
I live and work in South Africa and we are in the middle of an energy crisis. Random power failures (similar to the California experience a few years back) mean that most PC's and servers regularly have the power yanked, but for the most part there is no ill effect. I spent 7 years of my life servicing and repairing consumer PC's, which were regularly powered down without shutting down due to impatient or uneducated users, and I have never seen a disk that crashed for this reason. They ARE very sensitive to knocks and bumps, however, but even this tends to manifest as damaged bearings, or critical components being knocked out of alignnment, with catastrophic results. The old sight of a platter with concentric rings gouged through the magnetic coating is pretty much a thing of the past. In fact, I've seen more disks fail when brand new at the first startup, than after a power cut!
Similarly, modern filesystems are are FAR more robust and cope well with corrupted files and file-tables. Since the majority of the writing action which happens in a PC is to the swap file (which is re-initialised at every boot), or temporary files, which are normally deleted at shutdown (or at least, SHOULD be), no damage is done to anything which you care about.
Of course, this is not to say that modern systems are bulletproof - it DOES happen that a system becomes irretrievably corrupted, and it IS important to shut down properly for this reason, but the odds of damage are far less than many people make out - it's seldom enough that users become complacent. One of the most predictable statements from a user is "Um... what backups?"
In my experience, the most frequent hardware problems which a modern computer suffer from are failed power supply units (usually made as cheaply as possible), and seized cooling fans causing the processor to overheat and trigger the protective shutdown (or throttle down) circuits.
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Exactly, given the general crapiness of Windows, how would you know whether the problem is accumulated errors from improper shutdowns or just general Windows flakiness.
Jerry
At a bank I worked for in the '70s the operators used to stick a punch card with the volser on it in the groove between the clear plastic top of the disk pack holder and the base (used to hold unmounted disk packs). One day one of the cards managed to get inside the pack when it was mounted. Crashed that pack and the one in the drawer above or below it. These were IBM 3330's with 2 drawers per unit.
I'm _not_ advocating ignoring proper shutdown. I always try to shutdown properly. I just don't think it causes all of the problems some people tend to blame on improper shutdown. Especially considering the number of times I've had to power off W98.
Jerry
Release the hounds Smithers...
And aluminum comforters!
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