Web Cam help?

Okay so I am having a problem with my roommate. I don't know if he is coming in my room or not since I'm not home very often, and I've tried to find some software that will allow me to set my web cam up as a security camera but none have worked. If anyone could help me find some free software that can allow me to do this I would greatly appreciate it.

 

You don't need software! Try following this guide to just record what your webcam sees. Then, after a period of time, just watch through the recording and see if anyone walks in!

 

What guide? Also that would take up a lot of space as I will have hours of recording going on as I'm not here often. I just need to it capture pics or video when it detects movement.