It is strange - I've played Skyrim since 3-4 months and till yesterday it was all ok. Today the game crashes to desktop after the Bethesda logo. Skyrim logo shows, no menu and then it crashes.

Tried to fix it with the sound quality change, tried to load without any mods, tried to change the priority of files load - nothing helped. It was even before the Dawnguard install. I use steam version of the game...

Any idea why it happens ?

There are very very few mods that are compatible with Dawnguard and 1.7. You should not be using them until they are updated to make them compatible. Use the "verify game cache" option in steam to check for and replace any corrupted files. Also you should delete/remove ALL mod files in the data folder, as the game will still load files from this folder even when disabled in the launcher.