There are better ways, as Werner alludes to, but here is a quick way, if you don't need to use it often (since the box width must be tailored to the content).
I have stuffed it into a macro \boxit{<width>}.
\documentclass{article}
\def\boxit#1{%
\smash{\fboxsep=0pt\llap{\rlap{\fbox{\strut\makebox[#1]{}}}~}}\ignorespaces
}
\begin{document}
\begin{tabular}{| c | c c c c | }
\hline
Location & \multicolumn{4}{c|}{Rounds}\\
\hline
0 & 2 & 23 & 41 & 42\\
\boxit{1.52in} 1 & 3 & 24 & 42 & 43\\
2 & 4 & 25 & 43 & 44\\
3 & 5 & \boxit{0.81in} 16 & 26 & 34\\
4 & 6 & 17 & 27 & 35\\
\hline
\end{tabular}
\end{document}

If your tabular data is purely numeric, one could trim the height of the box to be symmetric about the numerals (the above solution uses a \strut which allows extra depth for descenders). I also make the box in red, with a thicker (1pt) rule.
\documentclass{article}
\usepackage{xcolor}
\def\boxit#1{%
\smash{\color{red}\fboxrule=1pt\relax\fboxsep=2pt\relax%
\llap{\rlap{\fbox{\vphantom{0}\makebox[#1]{}}}~}}\ignorespaces
}
\begin{document}
\begin{tabular}{| c | c c c c | }
\hline
Location & \multicolumn{4}{c|}{Rounds}\\
\hline
0 & 2 & 23 & 41 & 42\\
\boxit{1.47in} 1 & 3 & 24 & 42 & 43\\
2 & 4 & 25 & 43 & 44\\
3 & 5 & \boxit{0.77in} 16 & 26 & 34\\
4 & 6 & 17 & 27 & 35\\
\hline
\end{tabular}
\end{document}
