disabling a particular cell in JTable

hi,
my table has got four columns.
according to the value at 1st column i should add combobox to 2nd or 3rd column but not on both.
i used cellrenderer first.
but i couldn't select any values from the combobox(i.e)the dropdown box is not coming.
but the isEnabled() is returning true.
anyone has any idea abt this?
is any other way could i accomplish this.
please help me