Unresponsive and erratic trackpad in certain moments

I have a MacBook (late 2009), the white one.

Yesterday a few drops of water dropped on the trackpad and from that moment it has a strange behavior.

At first, for example when I switch on the Mac, it seems to work properly. But then, for example when I'm writing or when I'm using the trackpad itself, it starts to be unresponsive or to move alone.

Sometimes if I touch it with a finger, it zoom in (or zoom out) as if I touch it with tho fingers.

When these problems occurr, a little zone of the trackpad loses its sensibility.
But after a few moments, or if I restart the Mac, everything works properly.

Is it an hardware or software problem?
How can I resolve this?