Hi Tuan,
you can do very easy by modifying the skin1/customer/main/search_result.tpl.
Look for
Code:
<input type="checkbox" id="WHAT_EVER_IT_IS" name="WHAT_EVER_IT_IS"{if $search_prefilled eq "" or $search_prefilled.by_sku} checked="checked"{/if} />
and just delete the smarty Vriables around checked="checked". Then it should look like this ->
Code:
<input type="checkbox" id="WHAT_EVER_IT_IS" name="WHAT_EVER_IT_IS" checked="checked" />
Thats it.