I agree with Jay that it helps to strip or cut through the paint first. If you have that much paint buildup, chances are aesthetics are no longer a big concern - so don't worry about making some dings in the windows - they can be fixed.
But even then, the windows may not budge.
I have used prybars to get under the windows and levered the windows loose. This may be easier from the outside of the house because there is no sill to get in the way.
I have also used a hydraulic bottle jack.
When forcing the windows, you have to be careful not to apply too much pressure in any one spot, or the glass will break. If you can slip something lengthwise between the prybar and the window (like a piece of ¼" steel) it will help distribute the pressure.
You may have to bust out the window jambs (the strips that hold the windows in place - do they have a name? - it escapes me...) You'll have to replace them, but they're just wood.