Error 21 on start-up

My son's XBox has given the error 21 on start-up - so after reading fixes on this forum, I got hold of a copy of Slayers Auto-Install - burnt them onto a DVD-R and a CDRW (both burnt at the slowest speed). Initial start up would not boot from the CDRW or DVD-R - so I read that if you hook upa PC DVD drive - you can boot up with any CDRW/DVD media - done this no luck.. so now I am lost as to what to do next as all the suggestions have not worked. I assume I can just take the hard disk out of the XBox and ro-do it on my PC with all the XBox stuff... thanks