If you cannot boot up using the disc that originally came with your computer, its possible the discs are damaged, dirty or your drive needs cleaning. You can use a drive cleaner for that.
eplacement System Discs can be obtained from Apple's Customer Support - US (800) 767-2775 for a nominal fee. You'll need to have the model and serial number of your Mac available.
If you're not in the US, you may need to go through the regional Apple Store that serves your location to find the contact number - http://store.apple.com/Catalog/US/Images/intlstoreroutingpage.html
...one other thing, is your firmware up to date? if it is, then try this method:
Specifically... into single user (command+S at the startup sound)
fsck_hfs -ypr /dev/rdisk0s2