I was just going to post and say that they did. So it's not a problem on this site -- maybe on my browser?
It sounds like you have a form of adware that serves ads whenever you view websites to hide itself, and it can harm the reputation of a site if a user thinks it's coming from them, so glad you determined it's not just us! These can even appear as popups without the browser open in the worst cases, and some of the ads are tasteless to say the least.
If you've recently installed a new program, you may want to look down your list of installed programs through the Control Panel to see if there are any programs you don't use that sound suspicious or related to advertising/shopping. These often come attached to installs of free programs that get their money by letting these annoying software tag along for the install process. 99% of the time we are able to prevent the install but many people simply click next/agree etc. and don't watch for those types of programs. This is good practice, anyway.
I recommend you first try
Combofix as a tool to remove it. Almost every antivirus I've used is absolutely useless, and people often spend $50-100 for them and even pay for subscriptions to find that when they need it, it fails them, but combofix is awesome and free. Combofix will look at DLL files within windows for things that have been changed and remove the changes, which often fixes all problems with the machine. I have done computer jobs in the past where all I did was run this program and felt guilty to accept money for the job
If that doesn't work, try
Malwarebytes Anti-Malware which is good in general for cleaning up infections you don't even notice. It may at the very least give you the name of the problem, if it cannot remove it. Then, you may find a tool to remove that specific type of adware from your PC. Wish you luck, and let me know if combofix works. I like to hear that, for it helps me further recommend it to people as the best type of antivirus I've found.
I have had infections like this so bad that it was less time consuming to back up my data and install a fresh copy of Windows rather than spend several hours working on a fix for it. Ultimately, that will be up to you and your level of experience with computers.
Edit: I just noticed Combofix isn't working on windows 8.1 right now, for whatever reason. They likely need to update it. If you're using that, fall back to Malwarebytes and cross your fingers.