It sounds like you may have inadvertently installed some adware in your browser, which is causing ads to be displayed on your computer. There are two ways to rule this out:
1) Try a different browser, such as Firefox. Do you see the ads there?
2) If not, go back to Chrome and temporarily disable all add-ons and extensions, following the instructions here:
http://support.google.com/chrome/bin/answer.py?hl=en&answer=187443
Scroll down to the section called "Temporarily disable extensions."
If the ads disappear, you can try reactivating the add-ons/extensions one-by-one to find the culprit that's been injecting the ads.
Please let me know how it goes.