The best I could manage was having them open with notepad.
Open with Notepad++ was missing from the right click context menu and when I tried to select a program to open text files with Notepad++ wasn't listed. Even when browsing to the location of the Notepad++.exe file this wouldn't work.
Turns out something got buggered in the registry settings.
For the key...
this had been changed to some office type MS monstrosity.
I changed the value for the (Default) String and replace the file path to the path to where Notepad++ resides, in my case...
"C:\Program Files\npp\unicode\notepad++.exe" %1
note the "%1" refers to the file name being passed to the command.
This then allowed me to select Notepad++ from the open with context menu and then tell XP to remember the file association.
UPDATE: Something else stopped "Open with Notepad++" appearing so what I did was went through all registry keys searching for "Notepad++" and where an old file path appeared (I had paths for older versions of NPP appearing in "open with" shell commands) I replaced these with the correct file path for notepad++.exe.
Working ok now.