No matter what, you have to at least remove the sash to repair the defective spring. If you can discern the brand of the windows, you can hopefully get replacement parts from that company or one of its distributors. If you have generic non-tilt wood windows that had a simple spring (approx. 1/2" in diameter by about 12" long), there is a product you can get to not only replace the spring, but also improve the performance of the existing windows. It's a replacement sash liner (called "Window Fixer") from Quaker City Manufacturing. You pull out the top and bottom sashes, put the sashes in the left & right tracks (made in aluminum or vinyl), and insert the tracks into the sash opening. They hold up the sashes, as well as gripping them to make them a bit more airtight. My observation is that they don't work miracles in terms of air infiltration, but they do hold the window up as needed. Window Fixer replacement tracks (or liners, or channels - whatever you want to call them) come in about 12 different lengths, cost about $20-$40/pair, and can be found in some hardware stores. Someone recently mentioned Home Depot too but I've never seen them there. Curiously, they don't show up on a Google search. The manufacturer:
Quaker City Manufacturing Company
116 Darby Commons
Folcroft, PA 19032
Another possibility is Blaine Window Hardware. They've offered obsolete, hard-to-find and current window hardware parts for over 50 years. Check them out if you need not a spring only but a whole spring/balance assembly (select "tube balance" or "spring balance" at their website - they have a lot of choices). I do not believe they offer a simple spring-only though.