A few days ago seemingly out of the blue when trying to load XBMC it would completely freeze up the box and when trying to go the original dash i get error 21. I can boot into EvolutionX and do anything other than these two things and can't for the life of me figure out why this is happening.I have tried to completely replace the original dash files on the C drive using both slayers and by FTP'ing my C backup, and also tried downloading the latest version of xbmc and still nothing... any ideas?

How is xbox modded? Chippd/tsop/softmod?

Its chipped with an x-changer

1. Format X,Y,Z and clear E:\cache folder.
2. Can you list the contents of C drive? For msdash to work, it should have all these files
3. Where is XBMC located on the hdd? C drive or other drive (E,F,G). Try moving to other drive.

I may be on to something... I just tried using slayers to completely rebuild my HD and when I first tried out the MS dash and XBMC they both worked fine, but when I changed the system settings so I could FTP and then rebooted, I get error 21 again. Could my xbox be connecting to live (its plugged into my router) and patching my original dash? If so, how could I fix this?

Use a softmod like Krayzie's Ndure (gamesave style) or Kingroach Ndure with shadowC protection to prevent dash updates.

Yup apparently that was the problem, I used Kingroach Ndure just like ya said and It worked like a charm... thanks for all your help ldotsfan