That's your registry... if you simply deleted your RA2 folder without uninstalling, RA2 is still on the Registry and so, when you try to reinstall the game, the setup program detects the previous installation on the Registry and it will try to uninstall an unexisting game = bug.
I would suggest getting a Registry modification program (like CCleaner) and removing the Robot Arena 2 registry entry manually, then you will be able to reinstall. Don't uninstall your DSL version however...
Then you might keep a backup of a fresh Stock version of RA2 for the future, and it would be good to make that backup a .RAR or .ZIP.