Ok, I solved this. Couple of things. First, I mentioned I didn't have an All users\Documents folder. What I did have was All users\SharedDocuments folder. Well, when I looked at the menu path in the navigation bar of the the files and folders window, it did indeed say Documents, not Shared Documents as is displayed in the file tree below. Nice and confusing... Anyway, what I did was go to the top All users folder and changed all of the security permissions to Everyone and Full Control, which changed all the subfolders to the same. (I had also shared the folders previously as per instructions from my research.) Changing the security permissions did the trick, and SP3 installed without a hitch.