A minipage
or a parbox
or a box in general need a \noindent
to prevent the indention at the beginning of a paragraph.
\documentclass{article}
\begin{document}
\fboxsep=0pt
\noindent\fbox{%
\begin{minipage}[t]{0.48\linewidth}
left part
\end{minipage}}%
\hfill%
\fbox{%
\begin{minipage}[t]{0.48\linewidth}
right part
\end{minipage}
}
\end{document}
I am sure that Raphink knows the answer but you don't provide a minimal example to show the problem.
I used the command \frame
to demonstrate the left and right part.

EDIT: How Andrew Stacey commented here the solution with beamer
. beamer
provides additional features / environments like columns
which is used in the minimal example. For more details have a look at the documentation.
\documentclass{beamer}
\begin{document}
\begin{frame}
\begin{columns}[T] % align columns
\begin{column}{.48\textwidth}
\color{red}\rule{\linewidth}{4pt}
Left Part
\end{column}%
\hfill%
\begin{column}{.48\textwidth}
\color{blue}\rule{\linewidth}{4pt}
Right Part
\end{column}%
\end{columns}
\end{frame}
\end{document}
