AD's will popup on screen
It would appear you have some sort of malware. I'd consider one of three options:


Antivirus - May get rid of the virus without harming your legit files.
Fresh reinstall of the stock ROM - WILL get rid of any viruses...along with anything else you had on your phone. It WILL NOT fix any malware-esque behaviours that were built into the rom
Install a custom ROM - Like 2), will get rid of any viruses and the popular custom ROMs, for example Cyanogenmod, do not have any malware-esque behaviours. This option requires you to root your device and unlock the bootloader. Googling 'install cyanogenmod ' should tell you what you need to know.


I would try the solutions in that order to save on unnecessary lost data or technical ordeals.