Sweave does not parse \Sexpr{}

Hi,

has anybody encountered and solved the following
problem?

I continued work now on a different computer where I
have also used Sweave successfully before. I can run
Sweave() and it generates a .tex file. All code chunks
are processed and translated to Latex perfectly fine
but somehow only the \Sexpr{} tags are not parsed. 

This is an example .Rnw:
\documentclass{article}
\usepackage{Sweave}
\begin{document}
What is \Sexpr{3*3}?
\end{document}

And this is the resulting .tex:
\documentclass{article}
\usepackage{Sweave}
\begin{document}
What is \Sexpr{3*3}?
\end{document}