My Compaq Presario 2100 (actually 2104US) had a keyboard problem. Luckily I did some more searching before buying a replacement keyboard. Having said that, HPís replacement keyboard is sub $35 AUD so I spent more time on this than it the part was worth.
The solution showed itself by accident, I accidentally started iTunes and noticed that I wasnít getting any sound. On checking the volume control app on Windows I noticed my volume was set low. I inspected the volume controls on the side of the case and discovered the Volume Down button was jammed between the case frame. I couldnít get it repositioned manually from outside the case.
It turns out that KeyTweak was right, key 131 is Volume Down. Had I realised that sooner it would have saved me a couple of hours.
Anyway I followed the HP and compaq service manual to disassemble the laptop. Unfortunately the manual isnít very clear on removing the motherboard, but I didnít need to do that step. Once Iíd got that far I could reposition the rubber button over the motherboard switch and reassemble. My laptop is working as good as new now.