Then there are PCs that have been in the office for three years and shared by people long gone. Cleaning off the hard drive and starting over removes many past sins. Windows Millennium is now available, but people in business stick with what already works and long as it works. It's summer and time for thunderstorms. Your modem is vulnerable to differing voltage levels between the phone and power line grounds.
Surge protectors help.
Disconnecting your modem when you are not using it helps. Even if nothing gets zapped, there can be arcing in the connectors where your modem plugs into the wall and where the phone line enters your building. Arcing slowly burns wires causing increased line noise and slower modem speed. You can save costly troubleshooting time by the phone guy if you check these places before she arrives. Summer gets quiet in some industries, but apparently not in training.