Your dentists Xray machine isnt killing you, ruf_ctr2. The yearly maximum radiation reccomendation (in milliRads - a rad is 100 ergs of radiated energy absorbed per gram of tissue) is around 1000*your age millirads, or your age in rads.
Your dentists x-ray machine puts out around 2 to 3 rads per xray if it is even reasonably modern, and they all are nowadays what with the regulatory agencies and all. Those levels have been determined to be "safe" by OSHA, and the number of rads your computer puts out, even with the case off, are in the range of 4 millirads per year.
btw, even with the case off, you are exposed to a lot more rads / year just by going out into the sunlight (see mom, I told you staying in the basement on the computer was healthy for me). Going out at night is only slightly better - that gamma radiation from space is a killer, especially when it isnt slightly tempered by cloud cover, and the suns radiation emission levels are ridiculously small when compared to space radiation.