I can confirm that after installing Dawnguard, I was getting instant crashes to desktop after the bethesda logo video. After going ahead and uninstalling absolutely every mod I had installed. (Mods from both Nexus and Steam Workshop). It stopped crashing at that point. Heh... oh wait. I left the portal 2 space core mod installed and it runs fine.... You are going to have to uninstall all your mods if you want to play the Dawnguard content. Something just discovered... it may not be all mods that causes it to crash. Some mods may still work with dawnguard. Just a process of trial and error until you find which mod is the culprit.
