Slayers is an installation disc. It isn't an operating system, and you DON'T install Slayers on your Xbox. You use it to flash the BIOS, set up the hard disk, install a dash and some apps, and it installs a menu that says Slayers 2.7 on the background screen, but that's all.
It sounds as though your Xbox was chipped or flashed, not softmodded. If that's the case and you want to install a 750GB disk then you still don't need Slayers, you need HeXEn, because Slayers only works for disks up to 250GB.
So, you now need to work out if your Xbox is chipped / flashed, or softmodded. Does the Xbox startup sequence look like an unmodded Xbox? Are there any logos, or different coloured flubber? Does you standard dash screen show the BIOS version? If so, what does it say? What colour is the LED after the Xbox has booted?
This post has been edited by Heimdall: Dec 23 2011, 06:46 PM