Does anyone know about keystroke loggers?would be much appreciated?

I am suspicious of someone that i feel is a friend but some things he has said have made me wonder if he did something to my computer.Im wondering if he may have purchased software and put it on my computer to monitor my emails etc..Ive checked some of these sites that offer the software they say it isnt detectable.Does anyone know how I can find out if he has done this.Thanks for any response