It never seems to amaze me how much a gaming company understands the concerns of their customers and addresses them before you ever spend a dollar with them. Steam consistently wants customers to have a good purchasing and gaming experience. For those on a mobile device:

"Notice: Just Cause 2 only supports Windows Vista or Windows 7 and requires a video card that supports DirectX 10 or higher. Windows XP and DirectX 9 are not supported. Does not require a Games For Windows Live account."