...and I went into the Master Ninja Tournament screen (which apparently is an XBOX Live feature which I've never used so I wouldn't know) and it asked me if I wanted to update my Gamerstag or something like that and stupidly I hit OK and it started updating my dashboard. Now I cannot boot to anything except the M$ dashboard! How do I fix what I've done? Please help! Thanks.

OK, how do I make an evox disc so I can ftp into my box and copy over default.xbe then rename it to evoxdash.xbe? I think thats what I have to do, right?

Just search for making a boot disc. Its easy. I'd use craxtion though to do it. And I assume your problem was that your evox dash was called xboxdash.xbe and your msft dash was called msxboxdash.xbe. When ninja gaiden updated, it updated xboxdash.xbe with the msft dash so now you are still booting xboxdash but it is your msft dash, and your evox is gone. If you make a boot disc just copy it over naming the dash evoxdash.xbe and making sure your modded bios will boot it first.

you need a new C drive get it from the SlaYerS ISO