Roundup will work for chickweed, but I haven't had much luck using it on horsetail; the only thing that has worked for me with that is persistence in digging it out...
Roundup doesn't stay in the soil, and it won't hurt nearby shrubs as long as you don't get it on the leaves. Or at least that has been my experience. There have been a few times that I have felt quite desperate in the spring about all the weeds in the garden and sprayed with RU, and then planted my seeds and plants a few weeks later, with no apparent problems.
