Wikipedia has issued a warning to its millions of visitors. The headline says it all: "If you're seeing ads on Wikipedia, your computer is probably infected with malware."
The free encyclopaedia gives three examples of how ads can get onto the site, but only one involves malware. First thing's first: get yourself a solid antivirus solution like Microsoft Security Essentials or Malwarebytes and start scanning.
If your antivirus solution doesn't find anything, the second way ads get onto Wikipedia is via browser add-ons and extensions. One example is a Google Chrome extension called "I want this." To remove it, or any other such extension, open the options menu via the wrench icon on the top right, click on Settings, open the Extensions panel, and click the Remove button next to it.
The third way is your Internet service provider (ISP) may be injecting them into Wikipedia and other webpages. This is most likely the case with Internet cafes and "free" wireless connections. In this case, you can't do anything other than just switch your method of accessing the Internet. You could of course block the ads, but that doesn't solve the underlying problem.
"We never run ads on Wikipedia," a Wikipedia spokesperson said in a statement. "Wikipedia is funded by more than a million donors, who give an average donation of less than 30 dollars. We run fundraising appeals, usually at the end of the year. But rest assured: you won't be seeing legitimate advertisements on Wikipedia. We're here to distribute the sum of human knowledge to everyone on the planet — ad-free, forever."