my mate had this exact same problem (although before he rebooted it after doing that, he deleted some c drive files, making him my idiot of the year 2005). When this happened its seems impossible to get back in again but heres how i did it, i'll try and explain the best i can for you.
1) i hope you don't mind taking your xbox to bits, cause your gonna have to
2) get a program called xboxhdm (for the pc)
3) now you want to make a boot image of xboxhdm. you also want to get your hands on a stock c drive from somewhere (either a friends xboxes c drive, or download one somewhere or get it from slayers auto install disc).
4) to make the boot image, put the contents of an xbox c drive in the "c" folder in the xboxhdm folder. then start up a command prompt (go to run, and type cmd, then enter), and navigate it to the xboxhdm folder on your pc, and then type make-iso-win.bat (thats if your running windows). you'll see an image in the xboxhdm folder, burn this to a cd with nero (or whatever burning program you use).
5) now boot up your pc with this boot disc in, you get a menu. when this happens, you want to turn your xbox on and wait till it gets passed the flubber animation and then unplug the ide cable from it and plug the primary master cable from your pc into it (assuming you have taken your xboxes hard drive out for easy access). if you've timed it right then the xbox drive will have unlocked and you'll have access to it, if it doesn't work then keep trying till it does.
6) so assuming you've done that, how do you know its worked? well type in 1 and press enter. you'll be asked to type in xboxhd i believe, then you get another menu, so if thats there and theres no errors then its worked.
7) now you want to build c drive, so select that option, it will ask you if your sure, say yes. give it some time to do its thing, and bingo it should have replaced your xboxes c drive.

with any luck, the xbox will boot (onto the ms dashboard if it was a stock c drive), from here you can try running the exploit again and it should work.
OR you can use the same program to copy an xbox hard drive directly to anoher one, however that involves you getting another xbox drive, unlocking it and copying the contents over. if you want to here the details of how to do that (tired from typing now!), just ask. got any questions, just ask.
edit* i know what installer he used, i used it once, its not the best thing to use, it seems to be prone to not working on occasion. by the way did you have a "live" option on your dashboard before this happened?
This post has been edited by crustyteacup: Apr 21 2005, 08:30 PM