Uninstall Chrome add-ons and then uninstall Chrome
Uninstall iAntivirus and Sophos, install the free ClamXav and run it when you need too.
You shouldn't have more than one anti-virus running.
If your on 10.5 your machine might be infected with the Flashflake botnet which indeed runs advertising and may be blocking your Software Updates
https://support.apple.com/kb/DL1534
After running this make sure you Software Update fully. Apple has issued a security update for 10.5
If your on 10.6 or later, you might not be able to download the Security Updates to remove the malware, you will have to do it manually.
https://support.apple.com/downloads/
Harden your Mac against malware attacks
Once you rmachine is up and running on OS X, then download Chrome again, make sure pop-up windows is disabled and install Ad Block Plus or switch to Firefox which does.