Are you using Windows XP as well? Pro or Home? Is it fully updated with SP1 and Security patches? On the Microsoft site, you can get an updated version of Installer, but it is unlikely that that is the real problem.
You say "when I try to do a Replace and repair with the CD it gives messages that don't make sense".
Nevertheless it would be helpful if you told us what the error messages were. It's surprising how much useful info can be unearthed with a phrase or two of apparent gibberish.
Was Office XP installed when you got the PC or was it pre-installed with a manufacturers Volume Licensing Key? If not, is your CD the same one that was used to install Office XP. I assume that you have the Product Key. You could always do a full uninstall/reinstall. You wouldn't lose your work.
Questions, questions.... But they all have a purpose.