A question thats been asked millions of times no doubt...

Before I do my new desk I have to sort out some creaky floorboards
underneath my PC area.

I understand screws are the solution... should I be making new screw holes -
or perhaps putting in bigger/fatter screws into the exitising ones that may
not be holding.

Also I have one floorboard in particular which has 'gone down' into the
ground (i.e something underneath had broken off) - it has only lowered like
2cms and is noticeable - any idea how to fix this (i will have to lift up
the carpet before I can try anything - i am hoping it will be as replacing a
wooden block and screwing down...

Finally, are there any spray type things that might relieve the creaking
sounds??