there is nothing wrong with using all fan slots however its likely not required.
at a minimum you want one front, one top and one rear. the top is even optional depending on your system specs and temps.
putting more fans in is NOT going to cause it to heat up. that is just silly. you dont want all intake fans... but you should have more airflow coming in than out. perhaps a 60/40 ratio (since air will find its own way out through cracks, holes, etc)
I have never had good luck with side fans. I added a 120mm side fan to my PC and they temps of my GPUs (2 gtx560s at the time) went up 2-4 degrees overall.
My computer is on 24/7 and I have never had heat issues ever. Leaving it on is not a problem. Heat is really only generated when gaming or when the CPU and GPU are in heavy load.
Yes, everyone has a cPU cooler. Without one your CPU would burn in a few minutes. I use an aftermarket cooler, not the one that comes with the CPU, because my CPU is overclocked.
Ive never had a cpu cooler and it never over heat but ive had issues about my hard drive alot of times and my screen would turn blue after having my computer on for a few minutes
No problem. You are probably thinking of aftermarket CPU coolers. Every CPU comes with a 'stock' cooler. But, many people buy better CPU coolers so they can overclock their CPUs.
do remember that fan configuration may vary based on what you have in the case. a side fan over a stock cpu heatsink for example is going to help not hurt.
generally though a side fan is not required except for certain circumstances.
heat is always generated by a pc, even when it is idle. its only a matter of how much heat. leave an idle pc in a room for a few hours. it will raise the temp of the room 2-3 degrees from heat dissipation. eventually if you leave the pc in a confined space it could start getting high temps but typically its a non issue as most houses have airflow throughout them.
@op
all cpus have a cooler on them. the stock oem one is just small. i use the provided cooler on my i7-920 and it works fine but then again i dont overclock, i have a large case and i have enough airflow. sure my temps would be lower on a better heatsink but if i dont push my pc very hard and if my temps arent too high i dont feel the need to invest money in something that isnt going to help since i dont have a problem as is.
an unstable hdd can cause blue screen of death (bsod). sounds like a failing hard drive to me. i've had my fair share.
as far as resolving what fans you need... you can start out with 2 fronts 1 rear (and dont forget the psu has a rear fan). start there and most likely you are fine. if you have trouble with temps post back in this thread and we can help you out.