There's no Halo2 for PC. However, if you like the mouse and keyboard setup, there are adapters you can buy so you can plug a keyboard and mouse into your xbox and use in almost any game, not just Halo.

And the only way to lower your level is to lose. A lot. But if you do it on purpose (and in doing so affect your team's score), your teammates can report you for cheating, which can get you banned, so I wouldn't suggest it. Just keep playing and you'll get better with practice.