There are two possibilities here.
One is that you are building up a static charge when walking across your carpet, sliding off of or onto your chair... something like that. When you touch metal on the laptop you are discharging that static charge. Does it happen more than once? If not, it's this sort of issue. The laptop is already providing a ground, so adding a grounding wire won't help.
What should help is grounding yourself to something else, preferably something with a high-resistance path to ground, before touching the laptop. Discharging through the resistance will cause it to happen gradually, so you won't feel the spark. One megohm or so is a good choice (this is typically what's in the anti-static grounding straps). If you don't know what that means, get a friend with good electronics construction knowledge to help you.
The other possibility is that the AC adapter of the laptop is "leaking" AC onto the laptop's chassis. If you feel it every time you touch whatever it is that you're touching, it is this sort of problem. This is an extremely dangerous situation, and it will not get better on its own. The cure is to replace the AC adapter... Preferably before you ever use the laptop again.