XBox HD's are locked to that XBox with a security key, as well as the OS resides on the HD, so putting in any old HD won't work, only putting in the original HD would work, or one that has been locked with the same key. If the XBox seems to boot up, you might get lucky and be able to FTP into the box if you know the IP or run an IP sniffer and if FTP is enabled. From there you could disable the theme, upload a new XBMC, etc but you need to know what you're doing.
IIRC, there is a safe mode for XBMC too, but I can't recall how to enter it or if there is or not, been a loooong time since I modded original XBoxes.