Computer not going to sleep or suspend

I have a user running Windows 98SE on an I-will motherboard and Athlon 1.4ghz processor. His PC is not going into sleep mode (standy or suspend) after a certain amount of minutes set in the power control panel. We have enabled power management features in the BIOS as well as Windows 98, but no luck. The only thing I can think of is that he is connected to a APC UPS via serial port which is constantly polling the UPS for power fluctuation. And he’s running an external USB ethernet network devicewhich may be preventing him from going to sleep. Any thoughts or experience on this would be appreciated.