Spyware and google

I bought the Avast 4.8 pro for 1 Jear. I made a scan of the computer but the program not found a trojaner wich make me problems wen i use google. Often wen i startet a webside from google it opend a side with advertising, but not this side i want. I dont now very much about things and in internet a lot of programs are. but i read the information that avast has a spywarescaner include and i bought the program.
Who has an information how i can find this trojaner with avast 4.8.
thanks for quickly information.

but i read the information that avast has a spywarescaner include
yes it has
I made a scan of the computer but the program not found a trojaner wich make me problems wen i use google.
no security program have 100% detection

Check your computer for Malware with

MBAM http://filehippo.com/download_malwarebytes_anti_malware/
update and run quick scan, click the button “remove selected” to quarantine anything found, and restart

SAS http://filehippo.com/download_superantispyware/

Are cookies really spyware and are they dangerous?
http://www.superantispyware.com/supportfaqdisplay.html?faq=26

come back and post the scan logs here

Do you mean this?

http://3.bp.blogspot.com/_ZaGO7GjCqAI/SiUg0nwZjTI/AAAAAAAAQAk/y4zCdEK122g/s640/google-sorry.png