Sadly, there isn't.
Campaign, new game
http://www.moddb.com/mods/elite-campaign
If I understand you correctly, you want to play the entire Halo: Combat Evolved campaign on a trial version of the game. That is impossible. The trial version of Halo: CE was "packaged" with only the single-player mission "The Silent Cartographer", and the multi-player map "Blood Gulch". If you wish to play the entire campaign, you must purchase the full game at a store like Best Buy, Target, or online.
Well, Halo Custom Edition is the multiplayer version of Halo Combat Evolved PC. It doesn't have a Campaign, by the way. If you want to get Halo Custom Edition, install the installer and use your Halo Combat Evolved PC Key. The main advantage of Halo Custom Edition is custom maps. Halo Custom Edition is currently in Version 1.0.9.620
No there is not halo for the PC
I don't play Halo 2 on PC but my guess is that if it has campaign or story mode, you can play alone. If you wanna play multiplayer with other players, you can't unless you do local matches.
From what I know, there is no Halo Wars on the PC.
halo PC does not have a ranking system.
No. Bungie stopped producing Halo games for Pc after Halo 2
Yes. There are a lot of mods for Halo and Halo Trail on PC.
The mathematical operation of multiplication.
Yes Halo ODST's campaign is definatly going to be longer than the one in halo 3, the long campaign was one of bungies main objectives in the making of ODSTAnother answerer:No Halo 3 ODST does not have a longer campaign than Halo 3, it is only an expansion to Halo 3