Since the Windows 8/8.1 upgrade assistant isn't compatible with Windows XP I suspect you are not going to be able to install Windows 8 on that computer.
The following are the requirements for running Windows 8. As usual, Microsoft will state that these specifications will allow you to run Windows 8, but it will be painfully slow and potentially problematic as a result.
Processor: 1 gigahertz (GHz) or faster with support for PAE, NX, and SSE2 (more info) realistically you should have at least a 2GHz CPU.
RAM: 1 gigabyte (GB) (32-bit) or 2 GB (64-bit) again, realistically I would suggest 4GB of RAM on the 64-bit version.
Hard disk space: 16 GB (32-bit) or 20 GB (64-bit) that's unrealistic.
Graphics card: Microsoft DirectX 9 graphics device with WDDM driver. [color=blue]A must![/b].
I haven't checked out the specific sites suggested for the free trial, but a fresh installation requires a proper product code to be entered before you can do the installation. I would want to check into this before going forward with this.
The bottom line here is that you are dealing with a older computer which could develop serious problems at any time. The electrolytic capacitors on the motherboard breakdown over time and can cause the computer to be very unstable. The hdd is an electromechanical device which will eventually fail from either worn parts or failed electronics. I would seriously consider investing in a newer or new computer if you wish to upgrade.