Praise be to Allah.
The fuqahaa are agreed that it is not permitted to keep a dog unless it is for a purpose, like hunting or guarding property or other uses that are not contrary to the Shareeah. The Prophet (peace and blessing of Allaah be upon him) said: "Whoever keeps a dog, except a dog for herding, hunting or farming, his reward will decrease by one qiraat every day." (Reported by Muslim, 2948)
As far as keeping a dog to guard the house is concerned, Ibn Qudaamah said: "It is not permitted, according to the most sound opinion." (al-Mawsooah al-Fiqhiyyah, 35/124)
For more information on this matter, please see question #377.
As regards the najaasah (impurity) of the dog, the Shaafii and Hanbali ulamaa say that the whole dog is essentially naajis (impure). Some scholars say that any liquid excreted by the dog is naajis (not the dog itself). The saliva of the dog is extremely naajis, because the Prophet (peace and blessings of Allaah be upon him) said: "If a dog licks the vessel of any one of you, let him throw away whatever was in it and wash it seven times." (Reported by Muslim, no. 418).
Abu Hurayrah, may Allaah be pleased with him, reported that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: "The way to purify the vessel of any one of you if a dog has licked it is to wash it seven times, the first time with earth." (Reported by Muslim, no. 420).
We must ensure that Muslims continue to be averse to dogs, even in the midst of what the kuffaar are used to do and what some Muslims have adopted of their habits.
May Allaah bestow good on you and on us. May Allaah bless our Prophet Muhammad.