Delete/prevent keyloggers: I need to block highly IT sophisticated person(s) highly motivated to know my movements, plans, communication from accessing my communications. Can I avoid reinstalling the OS? That would not block future intrusions.

Question

I have exchanged email with this person, so my IPS is known?  There is no visible device attached to my laptop.  A keylogger could be installed electronically? 
I have not given my password or key to my home, but there might have been entry/direct access to my laptop (any place can be entered if there is sufficient skill and motivation).  I cannot lock up my laptop every time I leave home, but I always log out before leaving. 
It seems that something like Zemantec would be ideal, but apparently it is only available for Windows.  Is there something suitable for Mac?  There are keylogger apps specifically for Mac, so what are the anti keylogger apps?  I have spent hours searching this matter, but reviews conflict, and product information is not entirely clear.
What would be the advantage of installing OS X.8?  I don't understand Apple's explanation of the advantages of Gatekeeper.  I would reinstall the OS or upgrade the OS at this time only if there were considerable protection from keyloggers. 
I am sending this from a different computer that I have access to only twice per week, so I do not want to discuss what makes me think that someone has installed a keylogger . . . . and I am not looking for reassurances that I should not worry about it.  I am looking for factual information, so I can make a decision based on what is possible/feasible, including the possibility that there is nothing I can do about it, or that I should not send some kinds of information from my laptop (extremely inconvenient!). 
I hope an IT sophisticate who is patient with an IT illiterate can help.

Answer 1

There are essentially three ways that a malicious hacker can "get into" your system (whether we're talking about keyloggers, malware, whatever)
- physical access
- persuading you to install software which gives them access (this could be disguised as something else)
- obtaining remote access by "hacking in through the internet"
Unless you have a file-sharing tunnel set up through your router, and your computer's file sharing is turned on, and you have no password (all these things seem unlikely based on your email), the third option is so unlikely that you can effectively ignore it. Having said that, if you have not checked those things recently, you should do so.  Post back if you want help figuring out how to do that.
The latest versions of OS X are always recommended for people who are concerned with security. Not only because of the major features like Gatekeeper (prevents non-signed code from running, which effectively stops you accidentally running a malicious script) and XProtect (built in malware-checking), but because of the other security patches to the operating system and compatibility with third-party components like Java and Flash, which actually tend to have more known vulnerabilities.
Wiping the hard drive and reinstalling OS X, preferably updating to the latest version, turning on FileVault (the current version requires OS X 10.7 or later), seetting a strong login password, and setting a firmware password, would go a long way to setting a good level of security on your machine. Write down the passwords and hide them somewhere that you'll remember, or keep them on your person if you feel secure doing that.
Having said all that, if you don't trust the physical security of your house, then all this is largely academic.
Matt