You need to find DLNA client software for Windows XP. You should be able to download content via FTP from Windows XP using FTP client software eg old versions of Filezilla but it will be of little use other than for backup purposes as the content will be encrypted.
I was specifically talking about files downloaded by FTP. All recordings are encrypted by the Humax.
The Win 10 download mechanism you are using is different and as you have found does remove the encryption from SD files.
They're not decrypted by Win10, they are decrypted by the 1800T when using the appropriate access mechanism.
By following the instructions, you can download decrypted files to the XP system. Win10 is hiding the detail of what is happening, and just happens to be able to detect and use DLNA servers as network file systems (ditto Win7). DLNA was never intended for that, it was intended as a plug-and-play means to stream media from a server to a player.
But, if you know what you are doing, you can copy (well, stream really) StdDef and HiDef programmes from 1800T/2000T to Windows XP using DLNA. I use it all the time in preference to using an external USB drive on the Humax. Piece of piddle when you know how.
Unfortunately, my "easier way" was to understand what others have done for other Humax models, work out what was needed to do something similar for the 2000T and my Windows XP computer, and write a program to do it myself.
... but required quite a bit of research to get there.
OK, I've spent some time on this today. Managed to ftp over (using FTP Commander) the .hmt file and 'Foxy' it. The HiDef file is ready to be downloaded on the Humax. Just having trouble now actually getting the file from the box to my PC. Tried VLC and the instructions given
The only way I could do something resembling what you are trying to do is to download foobar2000 and the plug-in foo_upnp. This finds the Humax but refuses to play the items. (Maybe there is a way to make it do it - but I've only just found it). With a bit of pratting about you can find the details of a Humax file (upnp browser, find file of interest, right click, properties, details, file path). Copy and paste to open network stream in VLC and it should play. Or just use your Win10 computer instead!
Not surprising, foobar 2000 is an audio player. Oops!
This foobar 2000 thing can do the upnp inspection and the wget bit - even though it is an audio player. It allows the user to copy (wget) the file and save it locally. Then you can play it with VLC. (I like my own way - see #12 - based on your original Idea: "Foxy-max" PC Utility (developer needed). Unfortunately that's another Java program that is currently undocumented, in a state of flux and certainly not ready to be released into the wild).