Please backup any thing necessary so you can revert back if things go wrong,.
If you don't have the Windows 8.1 Installation disk then you need to create a USB/CD Recovery disk:
- From Control Panel ->Recovery and follow the steps.
Win+R -> then type msconfig and delete the entry for Windows XP from the Menu and set Windows 8.1 as default.
From Disk Management, delete the Windows XP partition.
Set your BIOS to boot from USB/CD then Use the Windows 8.1 Installation or the Recovery disk to boot up your PC, If you boot from the installation disk, click on Repair my PC first else Trouble Shoot->Advanced Option->Startup repair->Windows 8.1
This should create a new MBR in C partition and set it to active (Windows 8.1).
Take out the USB/CD then reboot.
NOTE: For Windows 8, the options in step 4 is: Trouble Shoot->Advanced Option->Automatic Repair