Based on the description of the flower, and the plant's behaviour in your garden (spreading), I'm fairly certain that this is Epipactis helleborine
, commonly known as helleborine.
Here are a few links, with pictures:
Connecticut Botanical Society
Orchid Society at the Royal Botanical Gardens (Hamilton)
Orchids of Wisconsin
It is worth noting that on the last web site, they describe the plant as being aggressive and invasive, or as you described, "popping up everywhere". Whether or not you need to be concerned about its invasiveness depends on your situation. If your yard backs onto a public park or woodland, I suggest monitoring the local area to ensure that it is not spreading into the native flora. If it is only spreading in your garden, then simply pull it out before it sets seed.