AppData folder has been lost

I have been using my Vista Ultimate for a few weeks and I have logged in
today only to find all my application settings have gone. In trying to
figure out what has happened I have looked into the AppData folder and found
that everything in there has a new modification date and all settings for all
my applications have been lost! Skype account, browser settings, everything.
Infuriating isn't the word!! What has happened? I have searched the web
and these forums but found nothing relating to this issue. FWIW I'm an old
Software Engineer so I know my way around a computer. I hadn't done anything
unusual in the last few days.

Any help or insight appreciated or any further information I can provide to
help diagnose the issue just let me know.

Regards

Grant