Can anyone tellme why, for a few of my favourite particle templates, I'm getting just moving blocks of coloured pixels in place of some of the particle ?

There are two examples below: Christmas Star (3D) - I believe I customised this from the 'Star Trails' template, but now, instead of the big bright star, there is just a block of pixels, surrounded by the small stars.

Another example is the one I call Gold Christmas. Another customised one, from a downloaded Directorzone title.
It now has only pixels floating around the screen.

I still have the original uncustomised template. But suddenly the customised ones don't work, as you can see.
What have I done wrong? Obviously it's some folder or file that I have deleted, but I can't remember deleting one.
Can you explain it? AND When I'm customising a template, where is the default customisation filed? Perhaps that's the problem, maybe it's going into the wrong area?

I ALWAYS have problems with the locations of files. Jenny