enable disable button when event occurs

dear all

i m  new to asp.net

I have 2 listboxes listLeft and listRight and 4 buttons , these buttons are to move data from one list to another,these are

>

>>

<

<<

with very clear meaning

I want that when listLeft is empty then > , >> should be disabld and

when listRight is empty then <,<< should be disabled . and as soon as the listRight gets any item then <,<< shoul be enabled and vice versa

I tried Java scrip for doing this ,but couldnt do it

please help

thanks in advanse;
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