I see a lot of questions returning each 5 minutes, please read this thread before you ask questions!
(all credits go to: [KL] Master Dan [Stormcats] for making this thread)
When you are ding a quest, and you can not find a door to be opened, or you want to find a specific chest, YOUTUBE IT!
Just typ the name of the quest in the search area, and you will find a way to finish the quests.
Bought Legendary, got vanilla:
Right click on Skyrim in your Steam library, go to Properties, click DLC tabs and see if the DLC's have there checkmarks.
Once Dawnguard is installed, you will hear rumours about the Dawnguard when you have reached level 10 from guards in major cities. It is also possible, that a courier will approache to you and give you a note about the Dawnguard. If your character is below level 10, or you want to find the Dawnguard on your own, seek out the Dawnguard Headquarters roughly east/southeast of Riften. You can reach it by a small cave through the mountains.
If you clean the main files, Update.esm, Dawnguard.esm, Hearthfire.esm, and Dragonborn.esm it should remain cleaned, but do not to clean Skyrim.esm. It is safe to leave the Bash tags and Delev and Relev