at the logon screen look at the bottom left corner and you will
see a button with an option to change SESSION, ie desktops, Gnome, KDE
or Xfce. Then the pop up asks if you want to make your choice the
default. Xfce is a goood choice for a low spec computer.

I recommend a superb publication by 'Computer Active' titled Ultimate
Guide to Linux (actually Ubuntu) you may still be able to get it at
Smiths or online. All aspects are simply explained with good quality
pictutres.