Only if they are on different machines. you can't run two os's at once.
(Before anyone else says , i know you can with emulators, but lets not confuse the matter to much)
Anyway i was going to say,
You said you had problems booting from cd on that old 400mhz machine. If you still have the original cd drive in it, it may not be recognizing the media. Some old drives have problems with cd-r and cd-rw, or even just the brand of media (some are better than others).
Also the thing with setting the boot order and it not staying might be the bios backup battery dying. One way of checking this is to see if the bios times stays up to date.