That depends, Jennie. If you purchased PSE on DVD, an installer disk for both PC and Mac were included in the box. In that case just run the installer provided on the Windows disk. If you're definitely through with your Macbook then it'd be wise to deactivate it by signing if off from the Help menu. You're allowed a max of two simulaneous activations.
On the other hand, if you downloaded PSE for Mac, then it'll only install on a Mac. In that case you'll need to purchase a new Windows version of PSE, either electronically, or on DVD, to install it.