I am recovering from a computer crash in which I lost Democracy 3 and Social Engineering. How can I redownload these programs?
If you purchased the games directly from Positech via their vendor BMT Micro, just use the same download links that were provided to you in the confirmation email you received at time of the original purchase. Your game keys are recoverable via the same method. 
However if you purchased the game from Steam or any other game portal, you will likely have to deal directly with them.