This is Reimu Hakurei's theme from the fanmade Touhou game published by danmaq, Touhou Mutsuigeki ~ Concealed the Conclusion
Original(s): G Free
(Track 7, from 大空魔術 ～ Magical Astronomy)
少女綺想曲 ～ Dream Battle ／ Maiden's Capriccio ~ Dream Battle
(Reimu Hakurei's theme, from 東方永夜抄 ～ Imperishable Night)
永遠の巫女 ／ Eternal Shrine Maiden
(Theme of Levels 1-4, Hell Levels 16-19, from 東方靈異伝 ～ Highly Responsive to Prayers)
So, I got a bit carried away with the opening. Can you blame me? Besides, it was too much fun anyway :D
Clips are from 【Touhou Arrange PV】Your Voice【東方アニメ 回路-kairo-】
http://www.youtube.com/watch?v=iQY_-UU9lDo
... I'm not going to write an extensive descriptive paragraph today; I think the dialogue speaks for itself. And for those of you who haven't figured it out, speech marks indicate that Reimu is speaking; dialogue without speech marks indicate that Marisa is speaking.