if you don't mind hacking up the wood a bit, take a flat scrapper (about 1" wide), a small prybar (check hardware store for a "jimmy tool" usually made by Hyde Tools, sometimes Stanley, about $7) a razor knife and a hammer AND GLOVES. get comfortable in front of window (on ladder or on the counter -- have good footing/balance) and look to be sure the locks are in the open position at least on the bottom sash. use razor knife to cut the paint where the window sash meets the trim (BOTH INSIDE AND OUTSIDE OF THE WINDOW IF YOU CAN GET TO THE OUTSIDE). use elbow grease! about 1/3 of the way from top of sash, put the flat scrapper blade flat on the sash and pound it in to where you just cut the paint with the hammer -- do both left and right sides--wiggle the scrapper loose and try the same at the bottom of the sash (between sash and sill) but towards the sides. NOW take the bent side (curved side)of the jimmy tool -- at the top of the bottom sash and pound the curved edge down into the rail (pocket) that the sash runs up and down in.
pry UP on the flat side -- dont be afraid to use pressure -- then do the other side -- enough force on prying up on the jimmy tool should be enough to pop the paint seal. If it's not -- try shooting a little WD40 down the sides where you just put the jimmy tool wait a few minutes and shoot some more WD40 -- then use the flat side of the jimmy and go to the bottom of the sash where you put the flat scrapper -- beat the jimmy straight down between the sash and the sill and pull it towards you -- do that at both sides of the bottom and then go to work at the top side rails again -- where you sprayed the oil. if it still doesn't pop loose, you may want to try going around the edge of the sash with the razor knife again and working the flat side of the jimmy tool into the sides, top and bottom again. You can also, carefully, lay the flat scrapper bladE on the sill and use the bent part of jimmy to pry up under the window handles. REMEMBER, YOU WILL HACK UP THE WOOD DOING THIS, BUT THERE REALLY ISN'T ANY OTHER WAY TO OPEN THIS WINDOW. KEEP YOUR GLOVES ON TOO.