Cut/paste shortcut, or shift key, mutes speakers

I'm not sure where this problem is originating - hardware/drivers (Inspiron 1525), operating system (Windows Vista SP1), or something else I've got installed.  But hopefully someone here can help ;-)  I've searched these forums, Google, Microsoft Support, etc with no luck.

I've been using Ctrl+Insert, Shift+Delete, and Shift+Insert as my standard shortcut keys for many years, on all sorts of hardware and under every version of Windows to date (except perhaps Windows 3 - if anybody can remember that far back).  I now find, on the system described above, that both the Shift+Delete and Shift+Insert shortcuts, in addition to cut and paste, also mute the speakers on my system.  This happens regardless of the application I am using the shortcut in, and is an absolute  'prohibited content'.

Anybody know what's causing it, and how I can stop that behaviour - and learning a different set of shortcut keys is not, for me, an acceptable answer ;-)

And just before I post this, I find that the problem may in fact be caused by the Shift key itself (the right-hand one is the one I noticed it on), rather than the combination, since my practice is to press the shift-key first, then the other key for the combination.  And it doesn't always happen, just mostly (I assume it gets reset on reboot), since I've just managed to turn it off - but I haven't the faintest idea how I did that!  But a couple more practice edits on this message, and in a textfile, and now it's back on again!

TIA,

Rob