Web Browser - changes default home page

This may be the wrong area to post this but thought I would try it.  My default site for my browser keeps getting changed automatically and it is usually some other search site involving some porn sites and so on.  How does this occur and how do I fix it.  I have MCAfee with the firewall and this doesn't seem to stop anything...I have scanned with spybot and adware and am continuously deleting adware and malware...not too sure where to go with this and would appreciate any comments or guidance.