That's a common symptom of a browser or search engine highjack. Try disabling addons. In Firefox Menu, go to Help>Restart with Addons Disabled. See if the popups disappear?
If not, go into Add/Remove Programs and look for any unknown or suspicious programs installed? Uninstall them.
Try installing
Chrome and see if the problem is solved. If so, then if still using Firefox, backup your Bookmarks and create a new Firefox profile:
- Hold down "Windows Key+R", and type or copy and paste Firefox -p and ENTER. Follow the prompts to create a new profile. Import back your bookmarks and reinstall any add-ons.