If you uninstall the trial in the normal manner (Control Panel > Programs and Features) that will automatically adjust any account details it has online.
The same goes for your current XP system, except its Control Panel > Add or Remove Programs. (I think I'm reading this right..LOL). By doing that first the account will already be decremented by 1 license.
Then run the MCPR cleanup tool on your new machine to clean up bits and pieces left behind and reboot.
You should then go to your online account and download the software from there or you could have done what I often do to make things quicker, Save the installer file to my desktop first, then I have it handy to install the minute I reboot.
Thank you - very clear instructions.
You're welcome. Good luck.