My suggestion is to switch on the old machine. be sure to install thunderbird from thunderbird.net Once you have migrated to Thunderbird, here are the steps to transfer to new PC:

Here is the link for moving a profile in Windows when installed in default location:

https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

here is the info in brief steps:

1. ensure thunderbird is NOT running on old machine
2. enter %appdata% in search window on task bar and press Enter key
3. this should open windows file explorer in Appdata\roaming folder
4. highlight the Thunderbird folder and copy to external media
5. ensure thunderbird is installed on second computer and NOT running
6. enter %appdata% in search window on task bar and press Enter key
7. you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it
8. start thunderbird and all should be there

Thanks very much. One question about the first part...

"Once you have migrated to Thunderbird" - How is this done please? After installing TB does it prompt to install from New Outlook?

thanks again