I don't want to add another HDD I have already 3 internals and one external 1Tb in total.

@ naquaada

I have 2 partitions in my 1st hdd in the 1st partition I have Vista which I will erase and on the 2nd I have Leopard and I want to have the 1st partition for Time machine. But it want let me use it, it doesn't even shows my other 2 internal hdd as an option.