I am using Presario 2500 laptop and having problem with on of the key in the keyboard. Even if I don't press += key it performs. So is there any way where I can deactivate my laptop keyboard and use an externel key board?

Your issue seems very vague.
It maybe a simple case of some dirty stuck under the key in question.

Anyway, a simple resolution is to use and external keyboard, this won't deactivate your onboard one, but should help.

If the key is stuck because of dirt then you should take it to your local PC reapir shop and they can clean it up for you.

For interest sake, do you have an up-to-date antivirus installed?

Yep I use spy docotor with antivirus