At times, we see an SL installation fail part way through. Sometimes this causes the SL installation to remain in "Programs and features" though it is not fully installed. Other times, it will not show in "Programs and Features" but attempts to re-install fail citing that SL is already installed.
The following registry key has been found to be a link to this issue. Removing it will correct the problem:
(ALWAYS make a good backup of the registry before modifying and only do so at your own discretion)
HKLM\SOFTWARE\Wow6432Node\Microsoft\Windows\CurrentVersion\Uninstall\InstallShield_{674B8EF0-2BB0-4D59-A7CA-E77BB34004C0}