My site looks exactly the way I want it to when I'm not logged in as a registered user. As soon as I log in, though (either as "registered user" or as admin), the width of the left column (left block) gets wider.

This makes the menu (in left block) look bad. Also, when text appears in the center block, it is flush up against the menu (i.e. no left margin at all).

I am using only one theme, and I haven't set the site up to allow for users to select a theme. So... I know they are using the same theme... "view source" shows that they are pointing to the same stylesheets.

I've searched the forums but couldn't locate anything that seemed to fit. Any suggestions as to what I might try?

Could you give us a link and a testuser so we can login and see what happens? I guess logging in triggers something else (extra block, or usermenu that aligns wrong etc)