Stopping a full installation after entering the product key.

You can let it fully install, since you can then also correctly uninstall. You are not doing anyone a favor with 2 decade old software though. Not only is it insecure, making them a target for phishers and other malware, it's probably very unstable on new Win10 systems as well.
If anyone needs Office, let them use OpenOffice or LibreOffice. For email, let them use Thunderbird.

Office XP will install and function well on Windows 10.  See  https://answers.microsoft.com/en-us/windows/forum/windows_10-windows_install/a-look-at-running-older-versions-of-microsoft/6faf72ea-254a-4c8e-9982-2c36cdb1936c  and I've done the install and use thereof myself.

However security is another problem.  If the machine isn't networked it'll be fine.

As for Open Office it has infrequent releases and patches and has been supplanted by LibreOffice.  I've had too many problems with LibreOffice (and Open Office as well) to recommend them for handling Microsoft Word documents.  Simple formatting is fine but if it gets complex you can have difficulties.

If I begin an install and the key is the right one can I then cancel the install?

Use a virtual machine i..e hyper-v virtual machine you can then power off/reset  the machine

the question is not so much as stopping the process, but with whether the process will cleanup after itself.
Kimputer and masq and David provided you with the options that are fairly straight forward.

if anyone is unaware having a free email account on live.com lets or any other of MS's free email addresses no matter how old, long ago. It provides you with an Online version of Office tools such as Word, Excel, .....

Get a personal account and you get access to a  subscription based  desktop version.

"If I begin an install and the key is the right one can I then cancel the install? "
XP Setup will have already formatted the drive space, copied across the installation files, rebooted into a pre-installation form of XP, on a fast computer this will have set you back around 15 minutes.
You can cancel out at that stage but the formatting isn't easily reversed.

At that stage in the install you could then try all the keys you have until one works, then you know it matches the install disk you are using.  Once you have one good key you'll need to go through the whole process again for the keys you haven't tested and you'll still have no idea about what the ones that got rejected are for.

If you run PIDGen you type in each key to the program and it tells you which Microsoft installation disk you need for it.
No need to partially install Windows

The question applies to office XP, office 2002. Not XP as in OS.

I thought nickg had a mix of XP and Office XP keys ("We have some XP and XP Office disc and some keys ") and needed to work out which match each other?  
Anyhow, offering PIDGen as the tool to solve the question at the top of the page.

Been a while since XP is install. Believe after entry of key, other options gad to be entered, I.e. Key validation did not begin install of is, thinking back, the XP office key entry might have been Rone step before install.

XP tests on new system would require the slipstream of sp3 along with several drivers........

We have an Office 2002 disc and a few keys. Someone wants to install XP and Office for the sole purpose of using Word. The XP disc has a key and the Office does not. We have 5 keys that may be for Office 2002 and maybe not.

Have you tried entering the keys into PIDGen?

Thanks