Unable to open encrypted files

Archived from groups: microsoft.public.windowsxp.general (More info?)

Something happened and I found I couldn't open encrypted files this
morning. The files still indicated (under properties, General,
Advanced, Details) myself as the agent who could openly access it.

I've had no problems over 18 months I've been on the pc, and all the
necessary virus/worm checks show everything is clean.

Is it possible I've been hit by a root-kit which may have replaced my
security rule?

Fortunately I had a backup and only lost a few non-essential files, but
I'm not encrypting anything else until I find out what happened.