Basic C++ Tic Tac Toe

Hey all, I'm making tic tac toe in C++ and have a little trouble. I have some code but it isn't with me right now. The problem is, I have to make it very basic. I can only use if-then statements, cases, loops, and the iostream and string libraries since that is all we have done at school. I have the player's turn but need some help on the computer so it doesn't just pick a random number and go there. It needs to be at lease semi-intelligent. Any ideas?

You could assign points.

example:
Your bot is o, and it's currently the bots turn.

x.x
...
.o.

highlighted in bold is a spot where your opponent could place his x next turn to win, so lets upgrade that amount of points
point table(most useful has highest number, 2 points are given for blocking a possible win for the opponent, and 1 point is given for opening up a win opportunity for yourself.)

030
010
101

so the bot picks the highest scored spot

xox
.x.
.o.

opponent places it in the centre...
new scoretable:
000
000
303

ah, two times 3 points he wins!

another example, where it would be a tie:
x.x
...
o..

020
000
011

xox
.x.
o..

000
000
012

xox
.x.
oxo

000
000
000

doesnt matter where it places it, coz it's a tie
could assign 3 points to blocking opponent, 2 to opening up a win situation for yourself, 1 for every possible move and 0 for not possible.