My Fiancee and I just had a great time in Windermere last weekend, apart from all the pavements in Windermere being covered in sheet ice.
I do NOT recommend going to Windermere in winter if you're not very able bodied, the council doesn't employee anyone to clear/grit the pavements, making them very dangerous, especially for older people. we're in our early 30's and fit and still had lots of trouble.
When we gave up walking ourselves and booked a Taxi, the taxi lady said that her mother had been housebound since Xmas as she couldn't walk outside!
The council has a lot to answer for, looks like they're useless.
Otherwise, once you get out of Windermere and go for a wander around the lake, it's fantastic.