When I had a suspicion that my landlord was entering my house I setup a webcam to monitor it from work. I did it the cheezy way. Use Yahoo messenger and start the webcam, then from the other computer you can view it. You can also set who can view the webcam. Works ok but you got to remember to set the computer not to fall in sleep mode.
There is software out there to do this, I just cant remember the name of any. I set up a traffic cam for one of our radio stations a while back and it works great.