IMac troubles

Question

I have an Intel-Based iMac running OS 10.4.11 Tiger. Although most things seem to work normally, any attempt to load OS 10.5 Leopard results in the error code, "System X cannot be installed on this computer." Yet System X is obviously already installed in the form of OS 10.4.11, and has been for some time.
Any attempt to run "Repair Disk" also fails because I cannot force a startup from any CD or DVD when holding the "C" key. This is true for the Tiger and the Leopard installation disks. It starts from the hard drive whether or not I hold the "C" key.
The DVD drive is working properly otherwise. I can read whatever is on any CD or DVD that I insert into the drive. I just can't start up from a DVD.
The first sign of trouble was when I attempted to load some MS Word documents into Documents to Go on my Palm. I immediately received a Fatal Alert on my Palm. Fortunately, my Palm backup was only seven days old. My Palm has been working fine ever since a hard reset and a reload of my backup.
Immediately after this incident I found that the documents that I had attempted to load were corrupted. Any attempt to open them in Word resulted in a #10660 error code. That was the same code as had appeared in the Palm Desktop before I reloaded it. I trashed the offending documents. But then I realized that the #10660 code was showing up in various parts of MS Office as well.
Twice I uninstalled MS Office, reinstalled it, and loaded all of its upgrades. Each time, Word has worked fine and Excel has worked fine, but PowerPoint does not work at all. Keynote presentations work, but not PowerPoint presentations, although I receive no more #10660 error codes anyplace.
I have run "Repair Privileges" and have been told that there were no problems with privileges. I've left the computer run overnight so that the automatic UNIX maintenance routines could be run in the wee hours. As I've already said, I'm not allowed to run "Repair Disk." I have searched for any software upgrades and made those installations.
In summary, this is where I now stand:
1. I still cannot run "Repair Disk" because I cannot run Disk Utilities from
the DVDs.
2. I still cannot install OS 10.5 Leopard because it says, "System X cannot
be installed on this computer."
3. I still cannot use the "C: key to force startup from any DVD, including
the installation disks for Tiger or Leopard.
4. PowerPoint is still totally inoperative, even though anything else in
MS Office works fine.
5. Keynote presentations work even though PowerPoint ones don't.
6. Everything related to Palm works fine now.
7. All other software seems to work normally.

Answer 1

Is it saying "System X" or "OSX"? The precise working for an issue can make a difference.
Are you using a retail installer DVD (black) of Leopard or a DVD that came with another machine (gray)? Only retail installer versions will install on any computer. Machine-specific versions only work with that particular computer.
I cannot force a startup from any CD or DVD
Again, precise wording is critical. What CD are you trying to start from? No OSX operating system (except a few issued with utilities) for Intel Macs has been released on CD. This suggests to me that you are trying to install from an old version that will not boot your computer anyway, unless you didn't mean CD. What other DVDs have you tried?
In general I am trying to determine if you are even using versions that should boot your computer. If you are sure you are then you can try restarting by inserting a DVD (say, your Tiger DVD that came with your computer which should work) and holding down the option key.
Do you mean "Repair Privileges" or "repair permissions"?
You can only repair a disc from which you are not booted. You need to boot from your original DVD that came with your Mac and run Disk Utility from that in order to repair your internal hard drive.