If your knees are shot the shoes alone won't help (although replacing old shoes can help in a sense.) Do get checked out if you think you have an injury, and work on building muscle around the knees. IMO a lot of knee injuries can be prevented or at least reduced by keeping the muscle around it in good shape.
That said, the shoe advice above is solid. I also encourage people to get at least 2x pairs of shoes (different brand/style) and rotate them instead of running in 1 pair repeatedly.
Spot on with the shoe rotation, as in the Winter one pair will always be wet.
If you`re running off road, I`d say 4/5 pairs, especially in the Winte, due to the mud and water.
I`m currently rotating 2 x pairs of fell shoes, due to the amount of mud.