With an untouched original copy...
NO!
I HAD a softmodded v1 Xbox, man, those were good days... but again, i was like 200lbs, so i guess not. I like me now more that i don't play videogames that much, haha
There are Halo, and Halo 2 mods, just google them.
The PC vertions YES you can.
Halo 1 has this called Halo: CUSTOM EDITION you can download it, and create your own maps, modify weapons and the AI.
And Halo 2 PC I think has the scenario creator BUILT in. So....
I think Xbox 360's are near to impossible to soft mod, and are easily detectable by microsoft wich will get you banned, so yeah...