Are old CPU fans are useless?
No they are not.
Here is my old CPU fan with a broken blade.
Find some broken speaker cable, any length will do.
Strip a bit of the casing off so you can use it with the fan.
Do the same with the fan.
Twist to two stripped wires together like this. Do this to all three of them.
Wrap some tape around the cables so that they don't touch each other and blow up (over exaggeration it won't actually blow up)
Plug it into the motherboard system fan.
Screw it to the inside of the side panel and test to see if it is working properly.
If it does work properly then reatatch the side panel to your PC and your PC will run 2C cooler