Can anyone tell me why as soon as a reach Primm at the beginning of the game the NCR attacks me.

That depends on what you did before you went to Primm.

killed the powder gangers

Are you wearing any powder ganger armor? If so, you're probably "disguised" as a Powder Ganger who are enemies of the NCR, so they'll attack you on sight.

This /\

nah

I had this problem i fixed it by equiping then unequiping powder ganger armor, what i guess happened is that when i first put on their armor then put my vault armor on that it said i was disquised as a powder ganger but it did'nt pop up anything when i switched back to the vault armor.

It should say you are no longer disquised if you switch to non-faction armor, if it does'nt put on their armor then take it off until it says you are no longer disquised.

I think that there is a bug when we remove those disguises:

If you switch them for another armor, their effect would not be removed, so you have to remove them first by hand then wear the other armor.

If you are talking about inside the casino, you should holster your weapon before you enter. If it's too late, you might want to wait a few days in game before you return.