The computrer so is not smart!
Sure it blocks you whenever you have two in a row.
But whenever it does its next move is random. So you have a fair chance of winning.
Fix this and u'll have a decent game. However it is only noughts and crosses and i don't know but i dare say theres millions of versions of that on NG. To make youurs stand out u'll bneed to add some kind of gimmick of thing to make it different form every other generic version.