Desktop reverts to "XP Style"

I prefer a Windows "classic style" desktop with a slightly enlarged
font. I have set up my desktop this way and used it for well over a year
without problems.

Over the last two months I have experienced a recurring problem. When I
boot up I find that my desktop and menus have all reverted to XP style
with smaller fonts, necessitating changed via the "appearance" tab in
desktop properties.

After applying changes, the desktop will then stay in this style for a
few days - anything up to a week - then when I boot again I am back to
XP style.

I have searched various websites and forums, many of which tell me the
check various registry keys and all seem to be as they should. The real
puzzle for me is why the change at random every few days.

I have not installed any software or made hardware changes recently. I
regularly scan for spyware / adware and I have a decent firewall and
virus checker. Running several scans for spyware and viruses has
revealed nothing.

Any and all help appreciated. Thanks.
--
Pat