Well, I have got a windows 2000 CD and a laptop which I wish to install it on. The laptop is running windows xp at the moment, but I want to put 2000 back on it - for reasons too long and complex to go in to. However, when I put the CD in it does not boot from the CD, have got it set as the first bot device in the BIOS, and the setup option is disabled if I run the disk from within windows xp.

There is all a tool on there to create 4 startup disks, however when I tell the laptop to boot from a floppy it just comes up with "BOOT: Disk I/O Error Replace Disk" then won't go any further.

Any ideas?

You really need to have a boot floppy, that used to come with Win 98 (borrow it from a friend if they have it). Enable booting from the floppy as a first priority. If you are able to boot, you have to delete the partitions on the hard disk and format and install your Win2K.
If you can't boot with the floppy, you can replace the hard disk and install the new(old) OS.

tip of advise... do not downgrade computer...

but if you have to... get a floppy disk that have a F DISK...to erase and partition the hard drive...