If you are online frequently you should regularly clean your PC with antivirus and with spyware/malware detection and removal tools. You should also use 'disk cleanup' at least monthly, to remove the debris and unecessary files which accumulate on your PC, but that's regardless of whether you're online or not.

If, after all that, you notice your PC is starting to slow down a bit, then you should defragment. There's no point in defragmenting unless you are suffering performance loss from file fragmentation.

File fragmentation occurs because your hard drive is divided up into myriads of little 'boxes' called sectors. Files are often bigger than the sectors, and need many of them to be stored in. Windows finds empty 'boxes' for your file to be saved in, and if they are not physically together your file will be spread in a number of locations on the drive. Of course the more you delete files, add files, open and resave files the more your files will become split up all over the hard drive.

Defragmentation finds all the bits and pieces of your files and moves them all so that each file is together in the one spot. That speeds up file access, of course, because the heads of the hard drive don't need to jump about all over the place to read each file.

But as I said, defragment only when you need to.

I don't know if it's true or not, but I've heard defragmenting too often can ware out your HD. I guess that would result in running defrag several times in succession.

If I can ask a question on this topic. I need to defrag my home computer HDD at some point, and would be grateful if someone used to defragging hard drives can give me an estimate on time for the job so i can get a good time to plan.

Its a 8 month old 200gb SATA HDD with 70gb usage. AMD 64 3200 processor 1.5gb ram.

I only ask because the last time I have tried on previous systems its taken over night to only do 10%. That had a 4gb HDD!

By the way, I would recommend disk clean up if you are online a lot. Opensource browser Mozilla Firefox actually cleans alot of disk space used when browsing on exit.

To minimise the amount of time spent disable all running programs before defragging. If your drives are accessible in Safe Mode you are best to defrag from there.

Right.
If you experience extremely long defrag times, or if the defrag program keeps restarting itself, that's usually due to other open/running programs or processes changing the contents of the drive's data or otherwise interrupting the defrag process. Defragging in Safe Mode is your best bet, because Windows only loads a minimum amount of programs/processes in that mode.

Can anyone verify this information? I'm deeply concerned about this because I defragment my Windows XP partition weekly to avoid waiting a long time later. I have several friends who are also doing this, and they may be in trouble as well.

If you are defragmenting on a weekly basis then you are certainly subjecting your drives to an extreme and unnecessary amount of mechanical activity. Remember it's not just the platters rotating on their spindle, the read/write heads are located on a movable arm which jumps about all over the place as well. The more use your hard drive is subjected to, the shorter its working life is going to be. That stands to reason.

Precisely what effect that has in terms of useful lifespan I don't know. I mentioned above that defragmenting should really only be done when you've had noticeable slowdown on your system which results from file fragmentation. Defragmenting on a weekly basis seems to me to be similar to the housecleaning conducted by a person with obsessive/compulsive disorder!

The more sensible approach is to, on those occasions when defragmenting is actually needed, set the process in motion when you are walking away from the system rather than sitting down to it

Once a week is probably too often. Most people will have a new Computer by the time a HD would ware out from such an action, but I have no clue as to what the current/contemporary problems you might have.

Ok, so the problem is the hysteresis of the arm, rather than an electrodynamic glitch with the platters?