If you kept the downloaded file(s) on your old computer (or copied it/them to a USB drive or burned to DVD-R, etc), you can use that to install DKS on the new laptop. You don't have to re-download DKS unless you didn't keep the original downloaded file(s).
As long as you are online when you uninstall the game from the old computer that will automatically revoke one install, or you can do so later on Securom's website (ie if you are not online, or are not able to uninstall the program). It doesn't matter if you do this before or after installing the game on the new computer, as long as you do not already have it installed on 3 systems (re-installing on the same system is automatically detected, and doesn't use up another activation).
In Windows Vista/7 you can use symbolic links to point to a file/folder at a different location, which most programs will handle just like accessing the original file/folder. One of the examples in this
howto article is having Steam games outside the Steam folder. As long as the drive letter for the external drive is consistent on each system, you could set Windows to have the game save files to the external drive. It would be safer to leave the saves on each computer, though, and run a program to synchronize them to the external drive (or online, via Dropbox or Windows Live Mesh, etc).
