I just bought Dawnguard and as soon as my character tries to enter Fort Dawnguard the game closes out. What should I do? I turned all the mods I had off.

Simply turning your mods off isn't enough with Skyrim. You also need to remove them completely, including any loose files they came with--and especially any mod-related scripts that are floating around in \Data\Scripts. Completely uninstall your mods and see if the problem persists.

Thank you for the advice, I will try that.