Have you seen ads on Wikipedia, then you PC is definitely attacked by virus. There are many cases reported in which many visitors to Wikipedia have claimed that they have viewed ads on the encyclopedia website. It may be noted that Wikipedia doesn't shows any ads on its website, instead it is run by donations from million of donors all across the globe.
Wikipedia's Director of Community Advocacy, Philippe Beaudette, wrote on its blog that "We never run ads on Wikipedia. Wikipedia is funded by more than a million donors who give an average donation of less than 30 dollars,".
Wikipedia has warned its users that their systems may be infected by virus or malware which will be injecting ads in Wikipedia website pages by infecting your web browser, so the users must clean up their system from virus or malware and also try disabling the web browser's extensions.