Are they wood or metal? Are they double-hung sash (top and bottom sliding), or are they a casement style with a crank, or a side-to-side slider?
Usually, on any window that's been painted shut, the simple solution is to take a utility knife and very carefully cut through the paint where there should be a separation between the sash and the casement. Be sure to sear safety glasses when doing this, because the tip of the razor blades can sometimes break off over really stubborn areas, and you don't want to catch one in the eye.
This can be tougher than it sounds, because some of that old oil based paint can get as hard as nails, and of course you don't want the blade to scar the window sash itself so you have to keep it steady. But if you're patient and persistent, you can break through that built-up paint. You'll probably have to do that both inside and outside the window, because it's probably painted shut from both sides.
However, if it's not working because of the mechnanism involved (like sash ropes that are broken), then that's a different repair. Let us know.