Optiplex GXL 5200 boot sequence

I'm trying to save a Dell machine that's badged as an Optiplex GXL
5200 from having to suffer the vagaries of running Windohs 98, by
installing a proper O.S. on it in the form of Linux.

My problem is that there doesn't seem to be an option in the BIOS to
boot from the CD/DVD ROM

Any suggestions please?