10.3.9 system file lost ?

[Hungry Horace]

I have a mac mini running 10.3.9. It has recently stopped opening all mac-original software (mail, safari etc) but still opens other software (firefox, opera) and runs very slowly. A friend who looked into this believes that something happened during a software update (crash/power cut) and that some library files have been moved/lost/deleted. (He could be right, I don't remember that, but the kids use it more than I do)
He suggested re-installing the OS, but of course I couldn't find the discs. I tried installing 10.4 but after a few minutes it said this was incompatible. Any suggestions, or should I carry on searching for the discs ?