I am just transfering a Linux user onto a new Windows 8.1 computer and this user was using Claws under Linux and of course wants to continue using Claws under Windows.
My problem is where to copy the Linux file to under Windows, a slightly different problem to yours, BUT we both need that same info :-)
So where are the files under Windows ? Hidden of course !! ( Typical !! )
As we can't assume Win 7 and Win 8 put the Claws files in the same place try this:
1) start Claws and note the name of the top mail folder, in my case "Claws-Sylpheed ( MH)" ( yes I've been using Claws since its original Sylpheed days :-)
2) Open a Windows Browser and Search for your version of "Claws-Sylpheed", eventually you should be shown where the Claws files are stored.
Something like this : "C:\users\you\AppData\Roaming\Claws-Sypheed" under Win 8.1 ( Win 7 may be quite different of course )
As far as I can tell "AppData\Roaming\Claws-Sypheed" are hidden directorys and not normally seen directly, however under Win 8 if you enter "C:\users\you\AppData", Win Browser should open these directories for access.
This should allow you to find, at least, your actual old eMail files.
Hope this helps.