In order to use Cisco Webex on Windows XP you have to
1) Download the modified v12 version from this thread, extract it with 7z in a folder you want and press escape only the first time you open it (download link from first post: https://www.mediafire.com/file/pjthbixelfylwhd/360EE_12.0.1592.0_Modified.7z/file)
2) Install this extention by going to this site: https://chrome.google.com/webstore/detail/user-agent-switcher-for-c/djflhoibgkdhkhhcedjiklpkjnoahfmg
3) Right click on the icon that appears up right after installing it and click on settings.
4) Add a new user agent string so that webex.com thinks you are logging in from a Windows 10 device like this one that works from me:
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.164 Safari/537.36
To do that, after clicking on settings, copy paste the above user agent string on the field "New User-Agent String", put a name on the field "New User-agent name" and a number on the field "Indicator Flag". I just put 1. Press ADD
5) Go to https://signin.webex.com just click on the icon of the extention, select chrome and then the name of your user agent string. The page will refresh.
If you want to deactivate it, just select chrome and then default. This will enable sites to see that you are using a Windows XP computer again.
Note: The sign in button does not work for me on webex.com with this version of 360 so i log in from this url: https://signin.webex.com/