How would you known if you overclocked your cpu? If you've OCed you know that you have, therefore you know that you've voided your warranty. You can kill your CPU if temps get to high, and therefore computer won't boot.
Depends on the CPU. Pentium 4 Presscots are fine at 50*C, Athlon 64s usually go to 40*C. P4s can go to 60*C but it gets uncomfortable. Same with Athlon 64s.
If you OCing, it best not to get caught by the people that sold you your computer.
I tend to check these things when some one that seems to be going through a LOT of computer parts on the one system.
I know if I don't check, normally the people at my suppliers or manufactures end will.
if you are going to OC your computer, make sure you restore all your setting back to default before calming warranty.
I have cause a few people out on switching dip switches on motherboard.
I had a net cafe / gaming lounge that was constantly giving me problems as he was OCing
OCer :- But the computer has always been like that.
ME :- Bull$#!+. I personally built that system, and i know that I did not have that dip switch set to that value. And dip switch don't just move by themselves.