computer goes online without my input

My PC keep going online without an input from me. I keep my PC on most of the time and the other day when the computer was offline, I was called away momentarily. when I got back the computer was on line and had went to some college website. First it was doing this every once and awhile now the intervals are more close together. I have scanned my system for Viruses and spy ware but they always come back clean except for some low threat cookies. To be perfectly honest about this problem, I found out my registry could be corrupted and not knowing which was the better Registry Cleaners I down loaded several. One of my initial thoughts about the problems I'm having is maybe there could be a conflict between these programs. even though my currently installed security programs is not detecting any viruses or spy-ware, I still think that having these problems could very well be valid. one reason is when I'm actually on line by my own choice I sometime get redirected from the site that I chose to one that I didn't. Please Help