From your description this sounds like a classic SID problem. You selected an option that didn't install shadow C, and now you've overwritten the real dash on the C partition, or some other vital files.
First things first - DO NOT UNPLUG YOUR XBOX FROM THE MAINS! If you leave it unplugged for more than a couple of hours you will get error 16 and have to hotswap.
If Slayers will boot (I doubt it, but worth a try) then put it in a drawer, and never take it out again, because Slayers won't help at all with a softmod. Instead, grab HeXEn (Google Heimdall's Xbox Engineering Disc), burn it to DVD-R at 4x or slower, and use it to update the softmod to Krayzie 1.1.1, then skip to the penultimate paragraph of this post.
If Slayers won't boot put it in a drawer, and instead use a post 2004 live-enabled game (Halo 2 or later) to restore the original dash. (Start to log in to live, the first thing the game will do is update the dash and in the process restore it back to stock. You don't need a network connection.) Then get hold of the Krayzie 1.1.1 gamesave and use AR to put the two save files onto your Xbox. Krayzie always installs shadow C, so your softmod should be protected once you get it installed. Mod the Xbox (select basic install - UnleashX dashboard). Take a copy of your eeprom from the Xbox and put it safely on your PC. Then take HeXEn and use it to update the softmod (all it actually does in your case is update the dash version and the dash config file).
If you want to have XBMC as your dash then you would use HeXen option 2.1.2 or 2.1.3 to install Kingroach NDURE 3.1, or options 2.2.2 to 2.2.7 to install Krayzie 1.1.1. Then select option 4.1. Install XBMC to E. Manually overwrite the XBMC installation at E:\Apps\XBMC with the latest version (delete the current version first, then replace with the latest version). Reboot and make sure that XBMC works when you run it as an application from the dashboard.
Once XBMC works as an application you can use the dashboard switching tool (either run from the default UnleashX dash or from HeXEn - Option 5.1.2.3. or 5.1.3.3 Switch to XBMC on E) to change the default dash to XBMC.