imac hard drive problem

my 2008 imac was having hard drive problems so i switched it for a western digital 500 gb 7200 rpm and replaced it myself. after putting everything back together, when i turn on the computer the screen stays gray and later says no bootable drive insert boot disk. i then insert the mac os cd and nothing happens. ive also tried booting from the disk whie starting the computer. i was wondering if i had to pre-install the operating system on it before hand.
All help is welcomed. Thanks

G'day and welcome to the forums.

Big job replacing hard drive in an iMac. Look for a 'how-to' over the 'net and try again. Also consider the cable may have been brittle, cracked and failed. Here is one guide:-


http://www.amfiteatar.org/content/view/155/57/lang,en/

You have to boot the iMac with your original install media or a retail version of whichever version of OS X you're using. Then use Disk Utility to format the drive as HFS+, GUID scheme. Install OS X or restore from your backup (Time Machine).

If you've done all that correctly and it still won't boot from the DVD, it may be that you're using the wrong DVD. Remember, it has to be either the original or a retail version.