Q: how do i block pop unders on my google homepage os 10.7 5?

From your Safari menu bar click Safari > Preferences then select the Security tab.

Select:  Block pop-up windows

Your Mac does maintenance in the background for you > Mac OS X: About background maintenance tasks

Avoid so called Mac third cleaning utilities. They can do more harm than good.

On no account install or use MacKeeper.

GlimmerBlocker will do the job.

Further opinion on it and how to uninstall MacKeeper malware can be read here:

http://applehelpwriter.com/2011/09/21/how-to-uninstall-mackeeper-malware/

MacKeeper have recently said that the uninstaller from here:

http://applehelpwriter.wordpress.com/2011/09/21/how-to-uninstall-mackeeper-malwa re/

now works.