This is the same as
http://cactiusers.org/forums/viewtopic.php?t=81. Looks like this is a problem among many users.
Things I have noticed:
When this happens there is a correspponding copy of cmd.exe and rrdtool.exe that don't return.
If you comment out the pert of thold.php that displays the image it works.
IE
<img src="<?php echo $config["url_path"]; ?>graph_image.php?local_graph_id=<?php echo $graph ?>&rra_id=1&graph_start=-32400&graph_height=100&graph_width=300&graph_nolegend=true">
to
<!--<img src="<?php echo $config["url_path"]; ?>graph_image.php?local_graph_id=<?php echo $graph ?>&rra_id=1&graph_start=-32400&graph_height=100&graph_width=300&graph_nolegend=true">-->