If Frank's suggestions don't straighten it out for you, try this:
Make sure you are logged on in an account with administrator privileges.
Open the Registry Editor (Start>Run>regedit).
Navigate to, and select, this registry key in the left pane:
HKLM\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced\Folder\Hidden\SHOWALL
Delete the value named CheckedValue in the right-pane....it probably isn't a DWORD
value.
To do this, right-click CheckedValue in the right pane, and click Delete on the menu
that appears.
Now create a new DWORD value called CheckedValue.
To do this, right-click in a blank area of the right pane, select New> DWORD Value,
name it CheckedValue, and press Enter.
Set the value data to 1 (0x00000001).
To do this, right-click the CheckedValue entry you just created, click Modify, leave
it set to Hexadecimal, type 1 in the Value Data box, and click OK.
Do not do *anything* else in the Registry Editor, just close it when done.
Post back with the results.