My secondary double glazing sliding windows run in aluminium track. In
the base of each sliding window there is a groove into which are
fitted two plastic slider pads, which sit on top of the track when the
window is in place.
See here: http://www22.brinkster.com/tykeshed/index.html (Scroll down
for a picture of how the slider fits into the window.)
The pad has a little spigot which sits in the groove in the base of
the sliding window. I have lost one of these pads and ask where could
I get a replacement?