Wow. That is quite the problem. If this is on a laptop (please update your system specs) then it could be that your shift key is a hotkey for volume down but if this is on a desktop running a legitimate Windows installation, virus-free then it's a bizarre problem.
I could recommend using a program such as SharpKeys to explore the issue. Sharpkeys is a GUI-based registry editor that enables you to change the functions of any key on your keyboard to that of another key. For example, on an older keyboard with no media controls I reconfigured my F11 and F12 keys to act as previous track and next track.
It takes a small amount of getting used to, but if you give yourself a few minutes to familiarise the layout and how the program works, then it's quite simple to use. SharpKeys