To win your sexy wallpaper, and I know you want one, you're going to have to furiously click your mouse until you've destroyed Princess Yushia's clothes piece-by-piece in three 30 second rounds. I'd tell you about the wallpaper, but that would ruin the surprise, now wouldn't it? So go do that.
If this gets you excited about Half-Minute Hero 2, you can check out the recent port of the first game on Xbox Live Arcade. No sexy wallpapers included.