In brief, because it has angular momentum in the opposite direction. In length: (Here we go...) The early solar system was a hectic, dynamic place. There was lots of stuff hitting stuff, and stuff clumping into bigger things, that made them suck up other things by gravitation. When everything settled, (not that it is settled - remember comet Schumaker-Levy 9!) some things were sort of the same... almost all of the angular momentum in the solar system is in the sun, and almost all of the mass, too. It spins every 600 hours (found here: http://www.geocities.com/saeles/planetsrotation.htm, where there is crazy detailed stuff about spinning, big, nearby things!). Most likey, Venus had some serious impacts early on, and ended up basically upside-down. Look at Neptune! Its angle of inclination is, like, 97 degrees or something... It's just as easy (for me) to think of it as being upside down (like Venus) with an angle of inclination of 83 degrees! There is a theory that Venus was a sort of eddy in the rotation of all the stuff early on, and started out backwards (my theory!), but I just made that up now, so don't use it for your homework... ;-)

Like all questions of history and prehistory, science can not give an indisputable answer to this question (assuming the question is valid...). The pure creationist will likely say "because the creator made it that way." Others will come up with a variety of speculations which can not be confirmed, and often can not be refuted by any scientific course of discovery. Did it come into our solar system from elsewhere and get stuck in orbit? Did it turn upside down? Did it form from a turbulent eddy in the spinning primordial gasses? All answers will be speculative. It may be possible to refute some.

It doesnt have to turn counter clockwise, it just is. It has been there for a very long time, it may have entered our solarsystem that way, it could be from large impats.

It may spin the same way as the rest of the solar system except that it has become 'tipped up' so that the original north pole now points south as it were (ie. an axial tilt of at least 90 degrees.)