Drop down menu .enable the table control for one specific value of the dro

hi

i have a screen where i have a drop down menu . below the menu there is a table control with columns .all the lines of the table contol is disabled .i need to know how to enable this line for just only one value of the drop down list .That is when i select one specific value from the menu the table control line should be enabled.also where should i place this code.

Thanks

Avik