Since you booted Slayer's it seems you have a hard modded Xbox. Usually they are set to boot from the C drive.
Physically the Xbox has the following drives:
C - Stock
D-DVD
E - Stock
F - Modded
G - Depends on BIOS
H-O - Mem card slots
X - Cache drive only
Y - Cache drive only
Z - Cache drive only
Q - A virtual drive created by XBMC
You cannot copy to the Q, X, Y or Z drives. All files needing to be updated are on the C, E, F or G drives.
I would get a copy of Evox from the Slayer's disk and put it on the C drive. It will probably need to be renamed Evoxdash.xbe. You will need the skins folder as well as the Evox.ini file. See if that will boot.
If you can get that to work, put your new XBMC files in the E:\Apps\XBMC folder.
Using the shortcut keys found in the XBMC folders on your PC, FTP shortcut.....xbe to the Xbox as Evoxdash.xbe and the shortcut.....cfg file to the Xbox as Evoxdash.cfg. Both these files go into the root of the C drive. That should boot up XBMC. Any changes to XBMC will be made in E:\Apps\XBMC.