You computer is infected! Buy some penicillin for computers asap.
Seriously, you have malware problem. Malware is a general term for virus, addware, spyware, etc.
Lots of computer pros on here will guide you through getting a clean computer again. Stay off the porn sites is good advice but like an STD, sounds like it's too late.
I use the following FREE programs
AVG or Avast for antivirus
SE Ad Aware for addware/spyware.
Malwarebytes is another good one as others have already said.
Tuco: When you have to shoot, shoot, don't talk.