Error loading [...] shell32.dll; access is denied

Hello all,

Windows XP Pro SP2, all updates, 2GB memory, full virus scanning and
firewall.

Yesterday, after the latest Windows update (and it may be
coincidental), a number of functions generate the following message:

[X] Error loading C:\WINDOWS\system32\shell32.dll
Access is denied

The system works generally but there are a number of items, especially
via Control Panel, that can no longer be accessed.

I have searched online for solutions to this, but do not see this
problem described. Details:

====================

CANNOT access either via Control Panel (shell32.dll access denied) or
via start-run .cpl files (nothing happens):

Accessibility Options
Add Hardware
Add or Remove Programs
Automatic Updates
ClearType Tuning
Color
Date and Time
Display
Internet Options
Java
Keyboard
Mouse
Network Setup Wizard
Phone and Modem Options
Power Options
Quicktime
Regional and Language Options
Security Center
SoundMax
Sounds and Audio Devices
Speech
System
User Accounts
Wacom Tablet Properties
Windows CardSpace
Windows Firewall
Wireless Network Setup Wizard

Also cannot access:
System via Windows-Pause/Break
Display options via right-click on desktop

====================

CAN access via Control Panel:

Administrative Tools
Folder Options
Fonts
Network Connections
Printers and Faxes
Scanners and Cameras
Scheduled Tasks
Taskbar and Start Menu

Also can access:

CTL-ALT-DEL Task Manager
CTL-ESC Programs and Documents
Device Manager via Computer Management window
Internet Options from IE (which I only use for Windows updates), but
some options (such as delete history and temporary files) generate the
same shell32.dll error.

====================

The shell32.dll is:
6.00.2900.3241 (xpsp_sp2_qfe.071025-1245)
8,486,912 bytes
October 25, 2007.

====================

On the Microsoft support site, I re-ran the WindowsXP-KB943460-x86-
ENU.exe, since this described a shell32.dll security issue. It
installed, but there was no change.

Where do I go from here?

Thank you,
Dennis

John,

Thanks for your suggestions. I have Ad-Aware, Spybot Search & Destroy,
Avast, and Comodo. The system is very clean -- it's my main machine
with all the critical stuff, so I don't need any malware or viruses or
other nasties, for sure. But I checked again, and it's clean.

So I ran Stebner's fix, which did a lot of repairs, but didn't help.

But then I thought -- wait, I've been using the somewhat balky
ZoneAlarm for several years and switched to Comodo last week after a
(trusted) reviewer pointed out how much more effective, transparent
and easy to use it was. It's hard to find out exactly what
applications are being dealt with, but I had no idea that it blocked
access to many system tasks automatically! Temporarily disabling it
allowed access to everything that was previously denied.

Now I have to find out where Comodo stores its list of applications
and permissions so I can allow access to the Control Panel. It's the
admin account after all, so I'm not sure why Comodo would do that.
Double-check for the double-dopey like me?

Thanks very much again.

Dennis