Parasite defense measures
Try malwarebytes to clean up your system:
http://www.majorgeeks.com/Malwarebytes_Anti-Malware_d5756.html
An avalanche of advertising schemes have gripped the entire internet.
Don't let the battalions of external parasites into your system in the first place. Take a solid defensive posture & stop these guys @ your 'doorstep' and keep 'em out. These steps will help, but methods (vectors) constantly change:
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INTERNET EXPLORER: Tools> Internet Options> Privacy> Advanced:
here check 'Override automatic....'; 'Allow session cookies'; 'Allow 1st party cookies' & 'Block 3rd Party Cookies'.
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FIREFOX: Tools> Options> Privacy: Check 'Accept Cookies from sites'
& Un-Check 'Accept 3rd Party cookies'
Firefox add-on's
Flashblock: https://addons.mozilla.org/en-US/firefox/addon/433
Add block plus: https://addons.mozilla.org/en-US/firefox/addon/1865
"NoScript" for Firefox will stop a lot of these adware/spyware & hijackers:
https://addons.mozilla.org/en-US/firefox/addon/722
Use this feature to block unwanted advertising re-direct:
With NS running, down on the far right corner is a blue 'S' with a red line through it: right click that> tick 'forbid -------' &/or mark as 'untrusted'.
(Ticking all may reduce interactive behavior; enable just enough to get what's necessary)
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Server based spy/adware, that tracks your surfing & builds a profile of your surfing with advertising conglomerate jackals: requires "Opt Out" and requires EVERY BROWSER ON EVERY MACHINE you use to be done...
Yahoo here: http://info.yahoo.com/relevantads/
Google here: http://www.google.com/privacy_ads.html
NAI garbage here: http://networkadvertising.org/managing/opt_out.asp
DoubleClick poison here: http://www.doubleclick.com/privacy/dart_adserving.aspx
(List of 64 Yahoo! co-conspirator advertisers they conspire with are listed here: http://info.yahoo.com/privacy/us/yahoo/thirdparties/details.html)
SpywareBlaster for system wide, realtime protection:
http://www.snapfiles.com/Freeware/security/fwantispy.html
When installing ANY new application, check the EULA for 'Toolbars' and/or other crud they try to slip past ya'.
**CRITICAL RECENT DEVELOPMENT**
Get this: the 'New' Yahoo, by default, allows your Yahoo! contacts computers, to send to whatever 3rd party software they have on THEIR computer, information about you! Can you believe that!!?
This is something you must end ASAP.
Look in your mail account settings (ya, it's buried very conveniently) and uncheck this box!
Go to your Yahoo! Mail: near the "Welcome (your name)", tick the 'Profile' link; next page, upper right corner, tick 'Settings'; next page, near the bottom is the box to uncheck.
And the deterioration of trust with Yahoo! grows...it's pathetic.
Good luck. :)