Need to install 360 on another computer that is freezing upon Windows Startup

Hi, I seem to recall reading somewhere that you can install 360 on up to three computers?  Is this true?  If so, I need to be able to run a scan on my dad’s computer that seems to be freezing either when windows load or a few seconds after it loads.  I’m not even able to boot it to SAFE MODE, it freezes just before even loading it!  Is there anything I can do for this computer with my copy of 360?