Navigation menu

hi everybody
I need some help for solving my problem. I’m developing a website, it has several pages(parents, and its subchilds) and I have create a menu using these pages.
Menu is following

Home AboutUs Projects Careers Services
| |
-> eCommerce ->OpenSource Development
| |
-> SCOR -> osCommerce
-> GANGES -> eCommerce
-> BCA -> Web Application Development
-> CMS -> Blog
| -> Forum
-> CHATLIB
-> BATELCO

My problem is I want to show specific menu item and its childrens in the specific page. For example
Projects page will only shows Projects and its childrens.
Service page will only shows Service and its childrens.

Note: I’m using a plugin “portfolio” in the project page, which shows my projects. Each time I have added/created a project it showed in the portfolio box and I just add it to the menu.

I just want particular menu item and its childrens, not page list.
Please help me.
and sorry for my bad english.
Thanks.