install Mac OS X

i have an error on my imac when it starts, so i need to reinstall the OS

but the imac doesn't start from the CD even when i press C at the beginning

what else can i try to reinstall the OS?

thanks

What iMac do you need to fix and what CD's are you using?
If you are not using the correct CD's for the model iMac they were designed for, they will not work. They will not work if they are to a different model all together. If they are a downloaded copy, I haven't heard of one that's worked properly.
If you are using a full stand alone copy of OSX, it should work.
Try inserting the CD into the computer while its on. Shut it off and restart but hold the option button. It should bring up the boot menu where you can choose the CD or HD (choose the CD) and hit the right arrow button.
It should restart using the CD. If this fails, I will have to say you're CD is bad.
Bryan

hello,thanks for answering. i'm fixing this computer for a friend. he has no computer skills or whatsoever.

i'm a network administrator but based on Windows.
i've done some stuff with mac and basically it's a computer as any windows based one.

i tried the Option button and it does nothing. how can i know which kind of mac computer is it so i might know if this CD's will work? i know the CD's are mac os X. they're 3 CD's.another friend made this copy for me.

thanks