John and Barry, thanks for the replies.
I ahve no problem with XP and think it is a very stable OS. It makes sense as you say, to not bog a little computer down unnecessarily with Win 7.
You have given me somwhwere to start. Thanks.
Michael.
P.S. Barry, what do you mean
"I regularly compile Stellarium from source " ? 
I just have it as desktop icon on my Laptop and run it from there. Hope that makes sense.