Have you ever registered it with Stardock?  If so, you can log on with the same email you used to register it and get the updates from them.

Have you ever played a Metaverse game?  If so, you can go to your endgame.sav and open it with a text editor. Manually scan for your Metaverse name and your serial number will be close by. 

Otherwise, you might be able to email Stardock Support and send them a saved game and they might be able to retrieve the serial number for you. 

<edit> Of course, if you want to make it easy, you can just buy it from Steam and download it for 20 bucks.

http://store.steampowered.com/app/202200/?snr=1_7_15__13

The way I figure it, your time is worth something. I'd personally take the easy way and save all the frustration and waiting. </edit>

I was giving him options.  If his was registered with Impulse, it's not so easy.  Often, it's less frustrating to find the information yourself rather than waiting on support.  I know.  I've had to deal with both Stardock and Gamestop support for a single issue.  Try it sometime.