Is there some kind of product that will protect curtains from the effects of prolonged sunlight?
You can simply hang sacrificial curtains (muslin, perhaps) behind the curtains you wish to preserve. There's also the distinct possibility that this is pure sales-hype, as I've seen many an old curtain that looked just fine.

In any case, even if you block ALL UV light, that will not prevent fading, according to this site: http://www.fsec.ucf.edu/en/consumer/buildings/basics/windows/fading.htm Most relevant paragraph quoted below.


  The most authoritative research on quantifying fading damage was done in the early 1950s by the United States National Bureau of Standards.  The U.S. Library of Congress undertook this research, in order to design a glass filter to protect the original copies of the Declaration of Independence and the Constitution.  Scientists found that blocking all of the ultraviolet radiation portion of the solar spectrum would not eliminate fade damage for most fabrics, but will slow down the rate of fading by a factor of about three. 


I didn't see "fabric disintegrating" as a common complaint when looking into this.