I searched for keyboard replacements and found that they are expensive (Sony service require 169$ to replace a keyboard). There are refurbished keyboards available in e-bay for prices around 70$, but I couldn't get one since I live in Sri Lanka. The price and tax is too much if I deliver something from US. So I gave a go by myself. Bottom line is I fixed all the keys for a price just around 1$ (one dollar :)
I thought of giving the steps I followed in case if some one else is in trouble like me.
Step 1: Identify the not working keys.
You can simply use a text pad to type the keys and identify the ones that doesn't work. To test function keys you can do many things. The easiest thing I did was to run a program in command line and then to press function keys in the same shell (note you should not run the particular program you chose as a background process). There each time you press a function key some kind of symbols appear. So you can see if it works or not. See this is a very primitive way but it works.
Step 2: Removing the plastic plate of the key
In notebook keyboards the plastic key plate is clipped to an underlying plastic support mechanism. See the following picture (the bad key in this case is F11)
Use a small flat screw driver to lift the plastic plate from top left corner and bottom left corner. Note: If you are trying to remove normal keys (like A, B, etc.) then you need to lift from bottom left and bottom right corners of the key plate. Once removed you can see it as follows.
Step 3: Removing the plastic hinges
You can use the same screw driver to remove the white plastic hinges. See IraqiGeek's blog to see more information. One advice is that don't force too much on these small plastic items as they tend to break easily. Once removed you will see something similar to the pictures given below.
Step 4: Peeling off the rubber button
The next step is removing the rubber button. It is little bit hard and needs lots of patience. The rubber button is fixed to the same plastic sheet which contains the touch points. So it is virtually irreplaceable :). The option I selected was to cut off the rubber button. You need a sharp blade to do this. I used a razor blade. Once again, be careful when you cut because a single mistake could damage the underlying carbon contact lines, leaving no option other than to replace the entire keyboard. Once you cut it off you can see it as follows (the second image shows another bad key, i.e. Esc key).
Step 5: Finding a replacement rubber button
If you can get away with this step then you are basically done with repairing. If you see the two images above you can clearly see the carbon touch plats (see the thin white "s" shape in the place where the rubber button is taken off. That is the margin separating the two carbon plates). The keyboard simply works if you can somehow find a button which will contact these two plates when pressed. There are numerous solutions you can find in day to day stuff which can be used as a replacement. The one I found was a simple push button. You can easily find a similar type of thing any electronic parts seller. See the images given below.
The first image shows the small switch I found and the next one shows the rubber button of it removed. You can remove it simply by pulling off gently from the sides of the rubber button (Note: be careful not to tear it off). The third one shows the underside of the rubber button and you can clearly see the carbon dot in it. This is the one which get pressed on to the carbon plates of the keyboard. The side view of this button is given below (in fact this is the same side view of the original notebook's rubber button as well).
The replacement button I found had this problem that the distance of pressing is smaller than that is of the original button. So when you fix the key plate it acts much like a touch button. I didn't like that feeling. So I simply shorten the rubber cylinder by removing a part of it and gluing the two parts. This is though seems simple requires a lot of patience. So drink lot of water before you do this :D and take a deep breath. Ah! one thing I forgot to tell was that not all type of glues can be used to bond rubber. So better to find a super glue which can bond rubber. In my case I found a cheap Indian made super glue (I think the name was Evlico or something like that).
Once you adjust the pressing distance and the button is ready to go you need to one more thing. The rubber button used here acts as a vacuum when pressed and sticks to the key board's plastic sheet. So I made two tiny holes symmetrically. That way it becomes very easy to press and feels much like the original button. See the images given below. I think you can see the tiny holes.
Step 6: Replacing the key plate
First replace the plastic hinges. I think you can do it by yourself. The advice here is don't push too much if the thing isn't attaching. You may probably trying to fit it wrongly. Then place the key plate on top of the rubber button and gently push from four corners (you can hear the "tick" sound as you press which indicates the correct fitting of the plate).
This completes the repair and following is a list of additional rubber button replacements you can use.
1. The calculator touch pads have a nice a rubber membrane with touch switches similar to the one I used.
2. TV / VCR remote controllers too have nice rubber buttons inside.
3. There are various types of rubber push buttons available in the market. So better to find a similar one that suits your machine.
If you have any doubt on the matter just drop a reply to this post.