--- Anybody with electrical know-how? --- I need help!
 

I am trying to connect a car 55watt HID (for an experiment) to a normal AC house outlet. I tried using a computer power source rated for 450 watts and it turns it on BUT it take a lot of tries (it wount turn on the first time).

SO

1- Can I add a "capacitor" or something to provide it with enough power to start it EVERY TIME?
2- OR what else can i use to connect it to the house? (It needs 12V DC and draws about 4 amps).

?

battery charger?

Car battery, battery charger or make sure that you have the PSU setup properly to use as a 12V source.

If you hae a membership to HIDPlanet check the PSU DIY thread HERE

add a 12v battery in there, even a really small gel cell would work, just something to work as a buffer, your power supply might not be designed for the high draw that those lights have when you first turn them on.