I have a rather bizarre problem. I installed 10.5.1 and restarted but the only way for it to boot is to leave the Install disk in, let the counter go down to (0) and it boots from the hard drive. Now when I don't have the install disk in, nothing happens, not even a "No Operating System" error, just nothing, and the post from the device IRQ table stays up, and the last thing it says is "Boot from CD:" which is always does even if I can boot another OS like Windows. Its like it sees the bootloader but just stops and doesn't want to load it up. Is there a way to reinstall the bootloader without reinstalling OS X, or a way to make GRUB take over instead or something along those lines? Thanks.
If you need to know anything about my system setup let me know.

When it boots from the HD it loads the bootloader code. When you see the "No Operating System" message, that comes from bootloader code (like Windows) too. Sounds like your disk is missing the bootloader completely. Assuming you did an MBR bootloader install, that is.

I was going to link to a post that describes what you should try, but the forums are so slow lately I can't get to it. Try searching for 'boot1h'. That is usually mentioned in threads where this sort of thing is discussed. It's a file you will need to copy into your disk's MBR. A million words have already been written about it.

EDIT: See here.

I sometimes forget how useless the search function is on this site. If you search for 'boot1h' you get back a list of threads with hundreds, even thousands of posts in them, 90% of which are in no way helpful.