I don't know what type of windows you have, but Double Hung Window Restoration is great for windows in older homes. I live in a condo building that was built in 1928. The windows are wood, double sashed, counter-balanced with leaded glass. Double Hung has already repaired/restored a few of the windows in the building and the condo association is contracting them to do the rest.
They first got some stuck windows to open and close again. This involved repairing the frames and even rebuilding the window mechanism in one case. Counter balances were also fixed where needed along with other small repairs to bring the windows into brand new shape.
If you have an older home I would definitely suggest Double Hung Window Restoration. One thing to note though is that they will not do any painting. They'll strip the existing paint and get the windows working again, but you'll need to find someone else to put on new paint.
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