After much experimenting, I found out how to repair the sash cords on Andersen Narrowline double hung windows, the ones with a spring box in the top sill (not the ones with hanging weights).
For those interested here are the steps:
1 - Get some sash cord. I used #24 braided nylon cord, it's the same size as the cord used by Andersen
2 - Go to: http://www.andersenwindows.com/common/pdf/0002051.pdf
and follow the instructions for removing the windows. You really have to pull on the plastic inserts on the left side of the window to get them out but they do come out. Don't be afraid to bend them, they won't break. Be sure to remove and tie off whatever sash cords you still have to something solid, otherwise they will spring loose with a snap as they slide out of the window lift point. They won't go anywhere but you won't want to be in the way.
3 - Remove the two top screws holding the top plastic insert and pull down. I could not get mine completely off but I did manage to pull it forward and down enough to get at the spring boxes in the top sill.
4 - Remove the two spring boxes. Each one is held in place by a single staple. Remove the staple and the box will come free (well, almost - on one side I had to pry it out - I found out later it was hung up on a protruding nailhead)
5 - Here you have a choice. You can either buy spring box kits from your local hardward outlet (Home Depot) or take the spring box apart and restring it. I did the latter. They require a little prying and then they finally come apart with a pop. They are held together by the two posts supporting the springs. On one side the posts are wedged. If you "unwedge" them with a pair of pliers, they come apart more easily. On the other side, the posts are firmly attached so the springs won't pop off accidently.
6 - Restringing is a little tricky since you're dealing with a pretty powerful spring but it can be done if you're careful. Measure the length of the old cords (mine were 33 inches for the bottom sash) and affix one end to the spring spool with a knot and work it into the slot. The other end goes through the outlet of the spring box and then to the white plastic slug with a hole through it. Thread it through the hole and then tie a knot. The knot is bigger than the hole and will keep it from slipping out.
7 - The real tricky part is loading the spring. I used a bench vice to hold the plastic slug, vice grips to hold the spring between pulls and wrapped the string around the spring spool, pulled it out, clamped the spool (so it wouldn't unwind), rewrapped the string around the spool and pulled a second time, rewrapped again and was done. Pull the cord out all the way to be sure you haven't overdone things since you need enough cord for the window to full descend.
I then put everything back together in reverse order and the winddow works fine. Now just 10 more windows to go, ugh!!