If you want a full answer, check out Schulman's Unauthorized Windows 95 and its sorta-sequel, Pietrek's Windows 95 System Programming Secrets. It's a sprawling

If you want a full answer, check out Schulman's Unauthorized Windows 95 and its sorta-sequel, Pietrek's Windows 95 System Programming Secrets. It's a sprawling exploration of the insides of this OS with the goal of answering precisely this question.