If you tried to put the old windows hard drive in your macbook you'd have to contend with:
a. Interface - is it the same interface the Macbook uses?
b. Size - it has to be the same size as the Macbook drive in order to even fit in the case.
Even if you get past that, you won't be able to boot it since a windows boot disk won't work on a Macbook.
If you have something on it that you need then Todd has the right idea - get an external enclosure.