Should I replace my old windows?

I get asked all the time if it is worth it to replace
windows in a house. Those beautiful single glazed wood windows in your older
home are not nearly as inefficient as you have been led to believe by the
advertising industry. Nor will window replacement necessarily be in your
financial best interest. How do I know this? I investigated how much it would
cost to replace the existing single-glazed wood windows (R-1) in my older home
with new, highly energy efficient double-glazed, low-E, argon-filled vinyl replacement
windows (R-3). Here is what I found out (numbers are only approximate values):