What can I install/download to stop adds and unnecessary links popping up?

[454]

I don't know why this has happened but for the last few days annoying adds pop up on every single site i go on e.g right now there is an add flashing saying THIS IS NOT A JOKE IPHONES TO BE WON, I can click "Hide these adds" but i have to do it every different site i go on to and every day.
Even more annoyingly, whenever I type something into google, a lot of links come up for thing completely unrelated to my search, e.g there when i typed in "yahoo ask" to lead me to here, all the following came up before the YAHOO ANSWERS section:

Download yahoo ask Instantly
Filesbunker.com
Looking for yahoo ask? Download yahoo ask For Free!
Watch Comedy Movies Online for FREE
http://www.viewster.com
Watch Free Comedy Movies Online. NoDownload. Free Streaming
Update your Media Player
Updatesoftnow.com
Your Player may be out of date, Approved for Windows
Software Update
Download.com
Your software may be out of date Update now, 100% Free.
Search Results
Lucky Mom Wins $23,455
JournalAndBlog.com
Clever Mom Uses Simple Trick to Win Read This Crazy Story to Learn How
Blinkx.com
uk501blinkx.com
Discover great videos at your fingertips very simply.
YAHOO ASK
explorednow.com
Find The Best Results Here Now.
Blinkx
blinkxgb.com
Great Videos at the tip of your fingers!

So annoying, i use google chrome, please help me to get rid of this problem, I don't know why it has suddenly happened?