Those of you who bought Modern Warfare 2 on Steam will not be able to play until Thursday.
Meanwhile, those buying a PC copy of Modern Warfare 2 from shops today can play immediately.
Both versions use Steam DRM procedures, but Valve has not explained why the Steam-bought copy requires an extra two-day wait to play.
Modern Warfare 2 was released around the world last night. The UK launch took place in London's Leicester Square, overseen by celebrities such as Vernon Kaye and one of the Black Eyed Peas.