Hello brother 

check out the same question posted in this link.....and have a look on my solution....you will get clear 

http://www.goiit.com/posts/list/analytical-geometry-hello-friends-one-interesting-question-for-990302.htm

Hello,

the product of slopes of two perpendicular lines not parallel to x and y axes is -1 bcoz they make certain angles with the coordinate axes and for them theta  is not equal to zero or 90 degree. but for x and y axes themselves or lines parallel to them , theta is equal to zero and 90 degree. therefore tan theta  is equal to zero and infinty and hence the product of their slopes is not equal to one.