I work for a charity who help individuals like yourself to improve the quality of their homes. It is all to common where people find themselves unable to maintain their property, when other thing need to come first, such as food and bills. It's a postcode lottery I'm afraid ...Many councils have funding for home improvements... many don't.
But if you dont ask you wont get... even if you own your property...as suggested previously.Try the Citizens Advice first, and ask if there is a home improvement ageny in your area who will help. They are usually part funded by the council, and can arrange to get the funds together to replace your windows, if you qualify ....you will have to allow them to see your financial details....( from what I see, the less money you have, the more you get. )
People dont realise but there are loads of charities out there with cash to give away...and unless you approach them you wont get help, and if you find this daunting, which most do. Get some one else to do it for you...we do it every day of the week.
There are benevolent funds available for almost every walk of life...so make an appointment at the Citizens Advice Bureau.
Good luck.