Is yours a "fault" state when it comes to divorce? Does your jurisdiction consider it to be "adultery" if you are separated, and he dates and/or sleeps with another woman? What would be your goal in gathering this intel?
Normally, I am in FAVOR of it, but that's usually very early in a sitch, when you're trying to determine the cause of their distance and strange behavior, so you can deal with it accordingly. If he's already been moved out this long, I'm not sure what you could accomplish.
But to answer your question, the keylogger that I'm familiar with -- eBlaster -- is VERY discreet, easy to install, and virtually undetectable.
Only do this if you think you can handle what you see, tho. It's often NOT pretty, and definitely NOT for the faint of heart!!!
