It's a feature. To save typing, TeXnicCenter additionally inserts \begin{ when environments are completed. This may be confusing if you don't know it. With this knowledge, it should be easy to use, as mindcorrosive said.
Though, you could change this behavior if you like. To do this, go to TeXnicCenter directory (C:\Program Files\TeXnicCenter for instance), change to the subdirectory Packages and open the file TeX.xml. It contains these lines:
<lxEnvironment name="document"
desc="Starts a new document"
expafter="}

\end{document}"
expbefore="\begin{"
/>
If you change the expbeforeline to
expbefore=""
then \begin{do would be expanded to
\begin{document}
\end{document}
But the feature just to expand do to the complete environment would be replaced. I guess you would keep it the original way, but if you're used to the other kind of code-completion you could adjust TeXnicCenter's behavior like described.
Note, you could add further .xml files in that subfolder to extend the code-completion capabilities.
code-completionorcode-hint. Kindly add it or a more appropriate one if you deem fit. Thanks! – Kit Aug 20 '10 at 13:15code-completionis appropriate, the more generalauto-completioncould be made an alias. – Stefan Kottwitz Aug 20 '10 at 13:20