I can't help with specifics but probably there is some residual files you did not delete that the installer is detecting. This could be a file in your preferences (I think that will be in your hidden library folder). With Mac applications it could also be a file in the receipts folder. Somebody who can be more specific will be able to better aid you if you tell us exactly which operating system version you are running. You may be able to use Spotlight to track down some Silverlight files but others may be more cryptically named.