Gorteen - if you can, check the hinges on each of the windows. Some hinges may be loose (simply re-tighten with a screwdriver or you could replace them if needed).
Make sure they hinges are clean of dirt build up, a toothbrush & a wipe of a damp cloth will suffice there.
Then, as PaddyBloggit suggested above - oil any moving part with the windows (and your doors as you have the oil out ). Ideally, should do this once a year. Windows should be easier to open & close then.
An alternative to oil of silicon is PTFE sprays. They dry out completely leaving a non-stick surface behind that won't trap dirt and grime in the moving parts. You'll get it in most bicycle shops, motor factors, etc..