Re: Carputer 101
Just putting a regular desktop in your car and plugging it into an AC voltage adapter is probably the worst possible way to make a CarPC.
If the PC is at all powerful it is very possible that your AC voltage converter simply can not supply enough power, so the overload light comes on. Is it one of the cheap ones that plugs into a cigarette lighter? If so...get rid of it it's useless.
You can make a voltage regulator work, you just need to buy a high current capacity one and wire it directly to your battery like you would an amp. Although again I wouldn't recommend this for anybody. AC adapters create a lot of noise and are unnecessary.
Ideally you want to power your computer with a power supply that runs off of DC rather than AC. Most of them have built in start up and shut down controllers that operate through a remote wire just like your amps or head unit.
References : chaunb3400, Team H&K, diminishedpower, pickup1, sublime06, sporty_drew, JRock10, Jando86, sundownz