why does this ad keep popping up on every website I go on?

[July]

On my opinion you are having the google redirect virus. If I’m right you’ll encounter the following problems:
•Suspicious adds keep popping up on visited pages
•You simply cannot navigate to specific websites
•Redirect a web browser to suspicious web sites each time the user enters invalid address or performs a web search like doing a Google/Yahoo/Bing search.
•New toolbars or Favorites are set up or added with desktop shortcuts and back links to webpages that you simply really do not want or knew about.
•Modify the important internet browser options, minimize the default safety degree and include unwanted or insecure sources on the reliable web sites listing.
•The browser retains redirecting you to the suspicious web site and all of the search engine results like Google search engine results and Bing search engine results are directing to porn websites.
•Monitor user’s internet searching practices, report addresses of visited web sites and mail gathered information to some distant server.

Once your browser has been hijacked, your PC system may be running with flawed security protection. From there on, browser hijackers also affect the system and installed software. Parasites cause the web browser instability issues, frequent errors and overall performance problems. Some reputable applications and browser plug-ins might also be affected by browser hijacker activity.
Concerning the solution, you can try this:
•Check Local Area Network (LAN) settings
•Make sure that DNS settings are not changed
•Check Windows HOSTS file
•Manage Internet Explorer add-ons. Remove unknown or suspicious add-ons
•Use TDSSKiller tool to remove malware belonging to the family Rootkit.Win32.TDSS
•Scan your computer with some anti-malware software.
•Use CCleaner to remove unnecessary system/temp files and browser cache
•Reset your Router back to the factory default settings

If you can’t handle the problem, I suggest using safe and reliable computer software as the 1 I've provided in my source. The Google redirect virus automated removal deals with malware infections that lead to Google redirect virus signs and symptoms and are so tough to detect and repair.
Good luck!