You have to change the Boot Options to boot from as follow:
1- CD-DVD
2- Hard Drive
3- Floppy Drive If present
4- USB...etc
check the manual of your Machine on how to access the BIOS, some machine by
pressing the F2 Key to get into the BIOS, some with a combination of keys.
Access/Enter Motherboard BIOS http://www.michaelstevenstech.com/bios_manufacturer.htm
I had an old Dell running win2k that would'nt boot from cd, allthough it was
set in the bios to do so.
After many attempts it finally booted from cd, I never found what caused the
problem