Plant looks like goldenseal except the flower is different http://www.flickr.com/photos/76284694@N02/9072300836/ http://www.flickr.com/photos/76284694@N02/9070076409/ I also thought it kind of looked like buttercup, but the buttercup I know has flowers that are yellow, not white.
Some Anemone species are cultivated and can be be quite aggressive in the garden. I don't think it is any of the species suggested by Andrey, perhaps Anemone canadensis or one of its lookalikes.