1) If you won't use your computer overnight - then turn it off since prolonged use will wear down parts. But also turning it on and off will put strain on parts - so if you are leaving just for an hour or less, then leave the computer on.
2) Defragmentations can speed up your computer a very slight amount (after intense benchmarking I found I get about a 2% speed boost in opening programs). Other than that, defragmenting really doesn't do much. So often I hear this type of conversation: "Person 1) My printer doesn't work. Person 2) Did you defrag?" I'm sorry but defragging won't fix any problems you have. I tend to do it every 6 months or so (and on the NT computers at work they are going on 6 years without a defrag and are running perfectly - since Win NT doesn't have a defrag included). Do it when you want, but it is certainly well overhyped.
3) I've run some computers for years without anti-virus programs, and other computers for years with them. I've had no problem with either - just one harmless virus which was quickly and easilly eradicated. Thus a responsible user will have an anti-viral program handy and won't be harmed in any way by running it at all times. However I've seen many people infected with about a dozen viruses a day (My in-laws for example, or one of my coworkers). If you click on everything and download everything, you will be infected - so an antivirus program is a must.
4) I've never been hit by a surge that caused damage, but my sister did. It fried everything in her apartment except her computer (microwave, TV, fridge, etc. were all goners). She had a $5 surge protecter on her computer that saved it. So no there is no reason to get a $75 one. The key is to replace them every 2 years or so - since all degrade quickly in protection while being used.
5) I've never used a can of compressed air for anything but the keyboard. Unless you are trying to get the best overclock possible, the small amount of dust won't cause harm. If you see a big dust bunny, then reach in and, pull it out of course...
6) Yes some parts may eventually fail. I've seen quite a few video cards become blurry (all the same model from the same manufacturer), I've seen a couple hard drives go bad, I've seen quite a few bad CD drives or bad floppy drives, and I've seen smoke come from a monitor. In each case, there was no user error - parts just have a finite lifetime. So the best advice is to be prepared. Backup your important files, keep your old computer for temporary parts if one part goes bad in your new computer, etc.
Basically it is all common sense.
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