Strictly speaking, you can't reduce grain - you can try and minise the effect using the variations of blur or noise filtering, but these will also degrade the detail in other parts of the image. There is reportedly a plugin for Photoshop which reduces grain, but I've not heard good reports.
You can however, reduce grain at the scan stage - Vurscan incorporates an algorthm for this which works quite well, as does the new Nikonscan 3 software - I guess others will follow.
It is "easier" to remove grain at the scan stage because the system has much more data to work with ... once that data has been converted into a tiff or bmp file, it becomes more difficult to distinguish grain from wanted detail.
Colin K. Work, Pixstel