Windows 8.1: Windows cannot be installed to this disk

(Those are just two partitions on one drive.)

Converting a data disk (without an OS) to GPT is usually painless, as only the partition table (containing start & end locations) needs to be rewritten – the partitions themselves do not need to be touched at all.

On Linux the gdisk tool can be used for this.

1. Boot a Linux CD (e.g. Ubuntu or Arch).
2. Open a Terminal window (if the CD has a graphical interface).
3. Run lsblk to check what disks & partitions are present.
   If you have only one, it's almost always "sda", but check anyway.
4. If the disk is "sda", run sudo gdisk /dev/sda to run the gdisk tool.
   It will warn you about a MBR partition table and will automatically convert it to GPT.
5. Use the p (Print) command to look at the results – if you want.
6. Use the w (Write) command to save the changes, or q (Quit) to cancel them.
7. Reboot.

(I think using the menu-based cgdisk would work just as well.)