I used once this software, but I can tell it works pretty well. It's called Kidlogger, you can find it at kidlogger . net.
You have to install it in your Gf's pc; don't worry about her finding it, because the program can hide itself from the menu and even from Control Panel, so she'd never find out.
It logs every activity she does in the pc, logs web adresses, takes snapshots of what's currently on the screen, can even record sound when it detects it being loud (you can set how loud), and take photos with a timer.
If she's always online, or anythime she goes online, the program will send the logs to your profile, and you can check them online. It also keeps a copy of each log, what's been typed, screenshots, sound recording, etc... inside the pc in a folder you choose.
It works pretty well, and if you want to know if she's been talking bout sex with someone else, you can input "hot keys", so that whenever she types a certain word, address, whatever, the program starts logging and taking snapshots.
I used it with my gf when we lived together bc I knew she was a lazy ass and spent all day long doing shit. I was the one with a job, so I wanted to know what she was doing.
I also wanted to know if she visited porn sites for the lulz and bc that shit turned me on, so I set different "hot keys" (porn, redtube, bondage, etc), so whenever she typed "redtube", it'd start taking screenshots more often.
Needless to say she was a fucking vicious girl, since every 3 min she'd get into redtube and brows for the strangest shit on earth; She had something going on with bondage, wincest, and got all that in pics. I also got some pics from her using the pc, but bear in mind that the lap camera's light will go on when it takes a shot.
Best of all: that shit is free. It is limited to how many logs and pictures can save in the servers, but if you have regular acces to her laptop, you can save everything to a USB for you to keep safe.