Bethesda Softworks
announced that the DLC
Dawnguard is available for PC via Steam.
QUOTE
The Vampire Lord Harkon has returned to power. By using the Elder Scrolls, he seeks to do the unthinkable - to end the sun itself. Will you join the ancient order of the Dawnguard and stop him? Or will you become a Vampire Lord yourself? Featuring an all new faction questline and locations, the ultimate choice will be yours.
The DLC can be purchased via the
Steam Store for US$20 in the USA and elsewhere, 14 quid in the UK and $25 in
Australia.