Invalid Boot disk error

This has been happening to my Dimension 4100. Despite the fact that my floppy drive hasn't worked for about a year and I've already removed it from my computer, it continues to say that I have an invalid boot disk in drive a. I tried booting from the windows xp disk yet it still delivers this message. I go into the BIOS boot information and it tells me that I don't have a primary hard drive, despite the fact that I do. I'm guessing that this is because the hard drive must have failed. So I uninstall this hard drive and then put a different one (that was showing up in the BIOS) in its place, and the BIOS tells me that there now is a primary hard drive. I restart (keeping in mind that the cd-rom drive, which I purchased new today, is set to be the first drive from which to boot) and I still receive the same error.

I've searched the forums for solutions to this problem, such as clearing the VRAM, yet this doesn't work. Anything that has to do with a floppy drive will not work as my computer does not have one. Any suggestions? Is there any possible way to truly force the computer to boot from the cdrom?