You should be able to reinstall Mac OS X, that should reinstall the entire OS whilst leaving everything like docs and applications where they are. Not sure if you need to press one of the keys to get it to boot straight from cd. If that doesn't work then as has been mentioned take it to the genius bar.
And I agree with Motion, he was spamming the other thread, merely saying things he doesn't like about an OS. I get annoyed by idiots talking shit about Macs but if someones just making a fair argument then wheres the problem?