I hope you are enjoying those double glazed windows, Helsbells.
I looked into replacing my 80 year old single paned windows but found that it is not at all cost justified. My current windows are in good shape and with care will probably last another 80 years. Instead of replacing, I have installed gaskets around the edges of the windows, caulked around the storm windows, and I am currently making window inserts to sit inside the frame based off of
these plans. All together, I am looking at $300 US to make what is essentially a triple paned window instead of the $4000 to replace windows. Granted, they aren't as sexy as the new double glazed, but they are good enough to pass my wife's "sniff test" on aesthetics and are very easy on my budget.
Cheers