Hi
First, remove Windows XP from your boot menu.
This is very important, if you delete Windows XP before removing it from the boot order your computer won't boot.
To do this the easiest way is to use EasyBCD but you can do it through the command prompt too.
I just like the visual interface that shows just what it happening.
Just tell it that Windows 8 is the default OS and remove Windows 7 from the boot options.
Also check the box that says to skip the boot menu at start up.
Next Reboot you computer to make sure it boots directly into Windows 8 without opening the boot menu screen. Try it a couple of times.
Once you are sure that it's booting into Windows 8 directly it's safe to remove Windows 7.
If the boot menu is on the Windows 7 disk don't format the disk or do anything to remove the boot menu from the drive.
I'm pretty sure that you can use EasyBCD to create a new boot sector on the same drive that has Windows 8.1 on it.
Here's where you get EasyBCD...
http://neosmart.net/EasyBCD/
It's very simple and easy to use.
Mike