When you delete a file in Mac OS X, and then empty the trash, the file is securely erased. That (ideally) means that the file cannot be recovered, regardless of your technical competency. When you delete a file in Microsoft Windows, and then empty the trash, the space that the file used is simply marked for reuse. If you have the technology, you can "undelete" that file until that space gets overwritten.
So the question arises: How do you securely erase files in Windows? The quick answer is: "There's an app for that". Over the years, my favourite app for this has been Eraser, but I just found a way to do it without an app. This approach works well if you want to recycle a computer and want to make sure your files can't be accessed by others: simply delete all your files (or your whole Windows profile), then run the command "cipher /w:F". It will take quite a while, but it will help you sleep at night knowing your files won't fall into the wrong hands.