An odd question regarding Internet Security

I was speaking with some guys in class today about Internet security, and a question came to mind....sorry that it is random, but I'm a curious person

Would it be possible for a person to hack into my computer (iMacG5 just so you guys know) and access my internet history/personal information... I'm thinking in terms of the hacker sitting at their desktop....If this is possible, how can I prevent it?

Safari has Private Browsing which you can enable if you don't want anything logged, such as where you've been etc.

definitely make sure your firewall is turned on
apple>sys pref>sharing

In a Windows environment, the easy answer is "yes." One could hide a script inside a .jpg on a website ("Click here for free access to Hot Babes Now") and upon activation it would be able to connect outbound and download additional code to log keystrokes or otherwise own your machine.

In an OS-X environment, you could still download a script, but at this point you may just be considered a 'carrier.' But any Windows user you would share that compromised .jpg with may then be infected...