PCStats has a super list of Beginner Guides. Here's my condensed version of what NOT to do to your PC:

1. Use your computer to hide dust-bunnies.
   
2. Don't worry that your computer is making more noise than usual, or making grinding sounds.
   
3. Try replacing that dead motherboard yourself.
   
4. Thunderstorms in the area? Just keep typing, it will go away soon.
   
5. Go ahead and see if that old RAM stick you found in a drawer will work.
   
6. Don't bother to turn off and unplug the PC while testing that old RAM stick.
   
7. The surge protector doesn't really do much anyway, don't mess with it.
   
8. Loose or dropped screws inside the case will just lay on the bottom, certainly not on an unshielded circuit.
   
9. Constantly plug and unplug your USB devices. It's fun!
   
10. Getting mad and throwing your keyboard on the wall is part of anger management.
    
11. Pet your cat before touching something inside the case.
    
12. Don't worry about internal parts jostling loose if you move or drop your PC.
    
13. Be sure to overclock your system. Everyone does it and you don't need that warranty anyway.