Su Root or MBentley...hrmm maybe it was MBrenley...actually I forget the actual spelling of his (her?) name....but those two seem to know an extraordinary amount on this subject.
In the mean time, an easy way to do it, is, after setting your shares, right click on "my computer" (on your desktop). Select MapNetwork Drive...Select a drive letter.....Hit Browse to find the specific shared drive/folder.....in smaller writing near the bottom, you can select "connect using a different user name." Choose this option, and input a username and password that is used on the computer you are trying to connect to.
Try that. I hope I explained it well enough.....hope it's along the lines of what you're looking for.