Energy-efficient windows do a great job of reducing drafts and increasing comfort. But the payback period, from energy savings, on the cost of new windows is long. So it's generally not advisable to replace old windows for energy-saving purposes alone. The most cost-effective time to buy high-performance windows is when you're building a new space, putting on an addition, or replacing nonfunctional windows. If you've got windows that work, you're better off, cost-wise, improving them than replacing them.
And that's pretty easily done. Repair any cracked panes and glazing, run a bead of caulk around window frames, and use side-mount sash locks to hold windows firmly in place. Block drafts through sash-weight pulley openings by using pulley seals. Use inexpensive window weatherization kits, available at hardware stores. If you have storm windows, use them in the winter. If not, have them made - they're far less costly than new windows.