alarm systems are going to be much lower draw, and less temper-mental..
a typical car should see a parasitic load at the battery of less then 0.5A. anything over that is considered high draw.
if you are really interested in keeping the pc on all the time, it would be in your best interest to use a second, dedicated battery for this, so that you will not kill your main starting battery, and be stranded somewhere. many of OldSpark's posts have info on how to power/charge a secondary battery, and there is also some info in tripzeros "[insert name of project] worklog" that details this..