If I delete pictures from my Dropbox account, will it also delete them from Win7 folders and iPad Photostream?
So long as the desktop/iPad is connected to your Dropbox account, yes, deleting them from Dropbox account online will delete them from your desktop/iPad.
Deleting photos from Dropbox will delete it from any synced folders like those on your Windows machine.
It will not delete any photos from your iPhone/iPad albums or Photo Stream. In fact, as of iOS 7 no iOS App Store apps are able to delete photos from iOS devices because there is no public Apple API to do this.