Cannot adjust volume from speaker icon or from Control Panel

I have been searching different forums where the speaker icon has disappeared from the notification area of the task bar, but have not found a solution to the particular problem I am experiencing. In my case the speaker icon is still visible but when I click on it, I no longer get the vertical bar which allows me to adjust the volume. I have made no recent changes to my computer and cannot understand why this has suddenly happened. I have gone to taskbar properties and tried turning the volume icon on and off but this has not cured the problem.

I have discovered the problem I am experiencing is limited to my user account only. I have one other named account and a guest account. The other user is able to adjust the volume from the speaker icon on the notification bar, and when I log into the guest account I find it is also working perfectly. However, when I log into my user acount it does not work. I have also tried adjusting the volume by going into Control Panel - Hardware and Sound - Sound - Adjust system volume but when I click on it nothing happens. However, when I go into the guest account I find I can adjust the volume from both the icon on the notification bar and by going through the Control Panel method.

I therefore suspect that my user account may have been corrupted in some way. I should stress that my problem is limited to my inability to adjust the volume in my user account. I do have sound, I just am unable to control it!

(I should add that I have run the Microsoft Security Scanner as I thought that my pc might have been infected with a virus but the scanner found no infected files).