With pdflatex you can use uarial (but this needs to run getnonfreefonts)
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[scaled=0.85]{uarial}
\DeclareTextFontCommand{\ba}{\bfseries\sffamily}
\begin{document}
Some words in the default font and \ba{arial} for emphasis.
\ba{abcdefghijklmnopqrstuvwxyz}
\ba{ABCDEFGHIJKLMNOPQRSTUVWXYZ}
\end{document}

Chances are the nobody would notice if you use Helvetica instead of Arial. This requires installing nothing.
\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[scaled=0.85]{helvet}
\DeclareTextFontCommand{\ba}{\bfseries\sffamily}
\begin{document}
Some words in the default font and \ba{arial} for emphasis.
\ba{abcdefghijklmnopqrstuvwxyz}
\ba{ABCDEFGHIJKLMNOPQRSTUVWXYZ}
\end{document}
