Look for the Multiplicity directory (in \program files\stardock\Multiplicity or \program files (x86)\stardock\Multiplicity probably).
From the start menu, type in "command prompt" and then right click on it to launch as administrator. From the command line, change to the directory with the old version of Multiplicity and install its service.
cd "\program files\stardock\multiplicity"
multisrv32.exe -uninstall
multipl.exe unload
erase multi*
Reboot and then you should be able to install 2.0 happily.
Kris