You currently have a single OEM license key that came with your Sony Viao laptop. If you sourced another copy of Windows XP, you would just be buying another license (key) and the installation media.
You could probably use the original OEM key, since you have kept it on the current laptop (one of the EULA's terms). However, it'd just be simpler to use the new key that comes with your new copy of Windows XP.
You can buy an OEM copy of Windows XP Home Edition in the US for about $100, you just have to search around. In the UK, it's about £60 (so equivalent).