I'll show how to protect a PC with a hidden switch. You'll be the only one who can turn it on and thieves will not directly be able to turn it on unless they take your pc apart and start 'fixing' it.
The first picture shows the reed switch which we will use. It's a glass tube with two contacts which are pulled towards each other making contact when a magnetic field is present. You can reed(read ;-) ) more details on wikipedia. http://en.wikipedia.org/wiki/Reed_switch
For the PC we'll use a read switch with a make contact.
The first step is to remove both side pannels from the pc housing by unscrewing the bolts shown in the picture. After that the front panel can be removed by pushing the front panel retainers together with your fingers to unlock them.