What's the best type of anti-virus or spyware to use for cleaning up my MacBook Pro? I recently switched to a mac from pc, so don't know much about my computer yet.

I recently switched to a macbook pro from a pc, and am curious what type of (if any) virus protection or spyware protection to use for it. I keep getting a message to Install MacKeeper 2012; but am not sure if it's worth paying for or not. I'd like some advice on what would be the best protection for my computer. Also, I recently had the new microsoft office installed; which has been having issues freezing up quite often when I use it. I don't know if clenaing my mac would help with the Microsoft office stuff, or not (or even if that would be related)...also each time I restart my computer; all windows applications open (word, power point, excel & outlook). obviously I'm not very tech savvy to begin with so any advice would help! I'm hoping this computer will last me thru the next 4 years of grad school & just want to make sure this computer is well taken care of to last that long. Thanks!