How serious is 'serious'? As in 'almost dead' serious? Military medics (as far as I know) follow a general rule, particularly when dealing with mass casualties. You fix the people who are the *least* dead first, then move along to the rest. Someone who is near death, will likely die. If you try to save him, someone else who might not have been that bad--like with a leg shot off--could get a lot worse in the time you try to save someone who has half their body missing. Just an example.

I have basic CPR and paramedic training, so depending how bad it is I could do stuff. Often the best thing to do is to in fact leave them alone, but if you can assess the situation properly you can determine if the position they are in will cause them more harm than if you were to move them. First thing you always do though, is call for an ambulance.

If there is blood about the place, stay away from it. It's fine to be a hero, but if you die months later of AIDS or something because you caught it from this person's blood, it's not so great. Unless you *are* a professional, in situations like this you should only intervene as a last resort.