Computer won't boot from DVD Drive

I'm in the process of building a new machine. I have it all done, however, I haven't gotten a floppy drive yet (which is the least of my concerns). Anyway... What can I do to this machine to make it boot? I'm trying 3 different OS disks... I had the Dell reinstall CD's for both XP and ME, plus a win 98 CD which didn't work. The computer DOES recognize the DVD drive, and it does ask to boot from the CD but ONLY with the win98 disk. When I tell it to boot from the CD, it doesn't do anything but give me a black screen with a dos curser. Anyone have any idea's? I'm looking for an XP CD, a real one, but I can't seem to find one yet, and I'm not sure that's my problem.

Also, I went ahead and took a 52x CD-Rom drive out of my dads computer thinking that might be the problem, but that drive doesn't do anything different from the other one.

Another thought is that the HD I have in the new machine was a backup from my old computer. I put it in without reformatting knowing I would have to reformat it before I can put XP on it anyway, so I didn't want to do it twice, is that the problem?

Any help would be greatly appreciated.