<p>I have a long form that the user has to fill out. </p>

<p>So I broke the form into logical units and created another user control for some elements (they will be reused elsewhere).</p>

<p>So say the form has these fields:</p>

<p>UserControl3.ascx</p>

<p>Username
password
email    -- usercontrol2.ascx
address    -- usercontrol2.ascx
city    -- usercontrol2.ascx
state    -- usercontrol2.ascx</p>

<p>So now in the codebehidn of usercontrol3.ascx, how will I access the usercontrol2.ascx's fields so I can write to the db?</p>

Short answer: yes.
In Mac OS 6,7,8 there was a substantial API devoted to this exact task. Lookup the "Resource Manager" if you are interested. Edit: So does the
ROOT
physics analysis package.
Not that I know of a good tool right now. What platform(s) do you want it to work on?
Edited to add: All of the two-or-three tools of this sort that I am away of share a similar struture:
The file starts with a header and index
There are a series of blocks some of which may have there own headers and indicies, some of which are leaves
Each leaf is a simple serialization of the data to be stored.
The whole file (or sometimes individual blocks) may be compressed.
Not terribly hard to implement your own, but I'd look for a good existing one that meets your needs first.