There is not a lot of "Adware" installed on a new Mac instead of all the junk a new PC comes with pre installed.
First is lock that Mac down and use ONLY a REGULAR account, not an Admin account on that Mac. Then to prevent a person from not getting to certain site without their knowledge (without installing software on your Mac) then you need to use the DNS service called
OpenDNS.com and get a free account to control YOUR DNS to block certain sites (like Phishing, adult, etc.) Plus you can assign person blocks to webs sites that you choose with a free OpenDNS account.
Then for the paranoid buy the program called
Little Snitch to monitor ALL outgoing traffic from you Mac.
Lastly you can get a lock down routine straight from the professional guide (for Panther(10.3.x) but you will get the idea) from the
NSA Security configuration. Warning this is locking a Mac down SUPER HARD.