Do you have a software with your mouse, for configuration...?
And what media player(s) do you have installed?
I know you said you didn't listen music but maybe your player has set your scroll mouse as volume shortcut, i've read an issue about Winamp...
And what brand is your mouse?
You can also try to update the mouse drivers, or uninstall and re-install it (the mouse)
re-re-re-edit... Is it a wireless mouse? I read a similar thread on a french site and this had been solved by taking off batteries "a certain time" then puttin'em back with the pc shutted off, then switch on the pc and let it detect the mouse...