You have to get into the BIOS setup program in order to change the boot device order. I'm not certain what key enables the program on your particular model, but it should be the "delete" key,
F1 key, F2 key, or maybe the F10 key. Once you determine the proper key, hold it down while the computer is booting and it will let you into the setup program so that you may set the CD/DVD drive first in the order.
Regarding the NTLDR error, you may have hosed the Master Boot Record when you tried to setup your harddisk to receive Linux. It sounds like you're attempting to complete a multi OS installation on one hard drive. If I'm correct, you need to go to www.crazytrain.com
and download Tom Rude's whitepaper on this subject. It's very well done and will walk you through the process.