Hi. I have a quick question. In the process of building my carputer (i'm using a HP tc1000 tablet PC motherboard) I hooked up a 12V power adaptor instead of the required 19V power one. It ran just fine until I figured what I have done

. I know that the reason why people use power supplies and such is because at crank, the power from the cig adapter exceeds 12V.
Since my carputer worked fine with 12 V, and it requires 19V, is it safe to assume that I could power it from the cig adaptor?
Thanks.
PS. I do have a carnetix P1900 power supply, but I need 12V for the monitor and 5V for powering USB and I do not have the 3rd 5V module.