PNG Problem on Windows 98

Dear all,


with the new version of R it is normally no problem to produce very fast
graphics in the png format.

In the office i want to work with R - Windows 98 is used.
There are french versions and english versions of Windows 98 second edition.

The png graphics are only on one computer correct.
On the other computers - the x-axis labels, the legend text and the "mtext"
(on the bottom) is missing. 
Attached are simple example files one in jpg (which is ok) and one in png
format.

produced with:

png("test3.png");
plot(1:100);
mtext(c("aaa","bbb"),c(1,2),c(4,1));
legend(50,50,"llll");
dev.off()

for jpeg with jpeg(......)


Any idea where the difference in the installation could be, like missing
files ore necesarry updates for the windows system? 

Thank you very much in advance for any help.      


Johannes