I replaced nine 6' x 3' windows in my breakfast room with Jeld-Wen Zap Packs, ordered through Home Depot. Basically a whole new window, with having to replace the frame boxes.
The only problem I had was on two of the nine windows, the existing box frames weren't exactly square. Simply had to remove a couple piece of brick moulding, square them up and finish the install.
I've used Zap Packs in a few other windows in my house as well, and have really had no other problems. Well, one time they sent the windows the wrong size, but that was because the HD guy wrote the measurements down wrong. HD honored it and re-ordered...at their expense, of course.
Randy