If you change the Boot Mode from UEFI to Legacy, you'll have to reinstall Windows 8. So you may want to image the entire hard drive before doing that, just in case you decide to undo... Unfortunately, Dell and the other PC makers are stuck with a lot of things being forced on them by Microsoft.  You might search these forums for other threads about installing Linux on the XPS 8500.  If you have Win 8 Pro or Ultimate, you have free downgrade rights to Win 7 Pro, which might be more tolerant of your dual boot arrangement. But you will have to install Win 7 in Legacy mode with Secure Boot disabled in BIOS. Without Secure Boot enabled in BIOS, you'd be no more at risk than you would be with Win 7 on any older version PC.
Dear Dell, I have just replaced my old Dell Dimension with a new XPS 8500 at great cost. On my old Dimension I was able to load Linux on an entirely separate disk. When I wanted to boot into Linux, I was able to change the boot priority and boot into Linux. However to my dismay I can't do this on my easily on XPS machine. I bought this machine to do with it as I wished and not to be tied down to using just Windows 8. From my understanding, this can be achieved by disabling the Secure Boot or whatever DELL calls it, though admittedly, without the intended boot-time protection. Is my understanding correct? What I am looking for from DELL is guidance on how to do this. I am not asking DELL how to install LInux but I am looking for guidance on how. to get my machine into a position to do this and any ramifications of doing so. I am installing Linux on a entirely separate disk so as not to disturb the Window's 8 installation. When I bought the machine, there was nothing on the website to indicate how locked into the Window's operating system you are. I think that DELL has a duty of care to help out customers who are in this position i.e being hindered into using the PC for other usages, which should be up to the consumer. I know that I am not the only customer  that wishes to do this and your help in this matter is greatly appreciated.
Installing Linux on a XPS 8500 