Intermittent access denied XP Pro

Hi EVerybody:

I've had a problem for a few days now that I can't find a definitive answer
for searching Google, etc.
When I log on with either my user name, (I have admin privileges) or even
the "Administrator" account (which I have visible on the welcome screen) the
computer does not recognize either as administrators and blocks access to
anything requiring admin rights. Right-clickiing on My Computer and then
clicking Manage brings up the Computer Management dialoge where Local Users
and Groups is covered by a red "X". Further, any attempt to open something
like WMI Control produces an "Access Denied" message.

Within a few minutes everything is fine, and the problem goes away: all
access is available.

Nothing shows up in Event Viewer.

Any help is greatly appreciated.

CN