GUID only ?

[blink41]

Hey !
I'm on the way to install my old leo distro (ipc) in order to get some informations needed for snow, but there is something that I actually can't understand : on every tutorial I've seen, there is always the need to set the partition scheme as GUID, not MBR. The fact is that I have some files on another partition on my HD (where windows is installed), and I was wondering if setting the partition scheme to MBR could be dangerous for my files...
Also, is there any way to install snow without a usb flash drive or usb HD ? I've seen there is a way with an hidden EFI partition, but haven't understoode anything...
That's all, Thanks in advance !