ifacmtg.cls (IFAC conference class file)

There are some problems to use ifacmtg.cls (IFAC conference class file)with harvard.sty because both are very old. The new natbib.sty came with 
MikTeX 2.5, that automatic  loaded by ifacmtg.cls  seem to be not compatible with harvard.sty.  To  solve the problems, you just
delete a line contained

\usepackage{harvard}

One more thing to do is , you have to change standard \cite to

\citep{Jo90} for                             (Jones et al., 1990)
\citet{Jo90} or \cite{Jo90} for        Jones et al. (1990) 

For  WinEdt people,  both \citep and  \citet  are still worked  with 
"Active String" when you type \citep{} and \citet{}.

for more details see Natbib manual version 7.4a.

By the way, harvard.sty without natbib.sty works fined.