500mA @ 12V is nowhere near enough. I'm not sure about the ratings of the Astec unit, but you will probably need a supply of at least 5A to run a PC. Mine takes about 9 surge and runs at 4.5. This is with the Arise computer DC-DC.
London, Ontario, Canada
Player: Cyrix 200, 32MB RAM, 10.2Gig Quantum HD, Onboard EtherNet/Sound/Video, Custom Lexan Case, Win95 Kernal w/Custom Player
Car: '86 Mazda RX-7 w/Basic Performance Upgrades