Inability to place windows on ground/floor level

Yes, standalone walls should be fine.  Are you building on a specific lot?  Have you built a basement beneath the area or have any other structures in the vicinity?  We did have something similar posted on the forums http://forums.thesims.com/en_US/discussion/921028/cant-intersect-with-walls#latest that I was able to duplicate in my game.  The user was able to use the bb.moveobjects cheat to successfully place doors that were blocked by that same error; you should be able to do the same thing as well.