I've seen several threads on this, but am submitting a new thread as suggested in another thread.
I have a school with 18 computers. ALL of them started the same problem at about the same time: at some point during computer use the taskbar turns gray and the volume icon, though present, does not work.
In some cases, I could solve the problem by rolling back the driver, but on other computers, this same method did not work. I do not have an antivirus program, but use DeepFreeze to keep my computer in its original state. The virus, if that's what it is, must have crept in at some point during maintenance. When I reboot, the computer goes back to its original state because of DeepFreeze, but the problem must be saved somewhere because it keeps happening.