On my widescreen monitor with a maximised browser it looks centered
. Not good enough? OK then, as Salaro says the only way to really play about with the layout would be to edit the ascx and move things around, the downside being any core updates will overwrite your changes.
I've looked at the page source and I can't see a simple way of hooking in some CSS so how about a really tiny edit to the ~/admin/manageusers.ascx to place a CSS hook. Find <asp:panel id="pnlUser" runat="server"> and add in CssClass="movepnluser". Create a CSS class .movepnluser in a suitable stylesheet with position:relative; margin-left:XXX; where XXX may have to be positive or negative, pixels or percentage, and can be determined through trial and error. If you do a core upgrade you'll need to go back to this file and make the edit again, but at least it's a small and safe edit.
I've pretty much written this off the top of my head, so if some of the detail is a little off please excuse me, if it's all completely wrong please ignore me
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Rhys