[R] Latex error about Schunk environment under Windows NT

[Cornec Matthieu]

Hi,

I am running the Sweave function on a Rnw file.
It produces a tex file with
\begin{Schunk}
\begin{Sinput}

\end{Sinput}
\end{Schunk}

and when I try to compile the tex file, I get a 
"Latex error : Environment Schunk undefined"

However, I wrote "\usepackage{Sweave}" at the beginning of my Rnw file
And the file "Sweave.sty" does exist in my tex files.

I am not experiencing the same trouble with my personal computer at home.
However I did the same steps. Is it due to Windows NT 
(my home computer runs under XP) and shall I install another latex package?

Anything would help.
Thanks a lot in advance,

Matthieu Cornec