What do you mean sharing different volumes
A partition is an area of hard disk reserved for use via an entry in the partition table of that hard disk. Each operating system may recognize and use partitions of various types; the 'partition' concept is system-wide, not limited to a particular operating system.
The partition table is a list of 4 possible partitions that is held within the first sector of the hard drive, and is normally interpreted by code within the same sector if that hard drive is booted.
In contrast, a volume is an operating system concept. DOS and Windows maps drive letters to volumes, and in this sense, a volume is any entity that has a drive letter mapped to it.
A non-removable local disk volume may be a primary partition, a logical volume within an extended partition, or a file within another volume that is treated as a disk volume via disk compression or other driver-level software.
A removable local disk volume may be a CD, diskette, Zip disk, etc. or may be something plugged into a PCMCIA or PC Card slot.
So sharing the MBR, drivers etc, except files and folders, will result in an unstable system.
I thought two operating systems that are dual-booted had to share a boot loader. That way either can be selected from a menu after the bios hands control over. When I got help from a user in Linux Mint's IRC help chat, he told me to install the grub menu on /dev/sda, Windows' location. Is there another way?
They can share a Boot Loader (Boot Manager) but there are other ways. You had not mentioned you were Dual Booting.
NB. The part about types of BIOS. Are you able to use the UEFI BIOS on your W8 installation?
but I can't boot into Windows XP without he drive that has Windows 8.1 installed. I think it's because Windows XP is using Windows 8.1's boot loader. I get error code 0x000000e.
I suspect the path \Windows\system32\winload.exe could not be found from the MBR on the XP drive.
Boot Manager. > http://pcsupport.abo...b/g/bootmgr.htm
MBR. > http://pcsupport.abo...sterbootrec.htm
As you are not going to use XP, suggest you completely disconnect that drive so will not interfere with any troubleshooting.
Is you original system a Dell Dimensions, XPS 410?
When you installed W8, Where did you get the MB drivers from?
Consider a fresh install of W8, get it up and running, then look at installing a Linux Distro.