Some keys on my MacBook are not working.

Some of the keys on my white MacBook (13-inch Mid 2007) have recently stopped working.

Here is a list of some of the keys that are not working: "1 2 3 4 5 9 0 r o p h m BACKSPACE".

Here are some keys that are working just fine: "6 7 8 q w e t y u i a s d f g j k l z x c v b n SPACEBAR".

The symptoms are consistent. No changes noticed over a period of two weeks during which the machine has been restarted several times and left turned off for a couple of days in a row.

The top case was replaced free of charge about 2 years ago due to cracks in the palm rests. All the keys on the external USB Apple keyboard that I am currently using are working fine.

I am wondering if anyone else has had this problem and if so what you did to solve it. As the machine is quite old I don't mind taking the risk of opening it up to fix it if necessary. If all else fails I shall take it to the local dealer and get a quote for them to fix it for me.