Computer virus... help!!!?

Hello, I'm Levi and i keep getting pop ups at the bottom right hand corner of the screen saying like "Danger Alert", "Stealth intrusion" or "virus intrusion" (Etc). and when i click on Mozilla Firefox this thing comes up that says "XP Total Security- Unregistred Version" then it and randomly scans my computer and says i have 27 viruses and if i want to remove them. i click remove and then it wants me to pay to remove them. so i scan with avira antivir control center or McAfee Security Scan Plus and they both say there is no virus in my computer. and when i am on facebook and trying to upload a picture nothing comes up when i click upload. and the sound comes on,on some sites and on others it doesnt and it doesnt work when i use my flashdrive. what is going on and what do i do that wont cost anything? please help!!!