Administrator Rights Lost

I’m using a Windows XP system since long time and never had any problem with the same. Today morning I ran windows updated and installed few important ones from them. Installation went fine without any problem but after rebooting the computer I’m not able to start any application or program on this computer except Control Panel window. Whenever I try running anything I received the following error message:

"Windows cannot access the specified device, path or file. You may not have the appropriate permissions to access the item."

Can anybody please tell me what is wrong? If you need here is the list of updates installed today:

Windows XP Root Certificates Update
Windows XP Update for Microsoft Core XML Services (MSXML) 6.0 Service Pack 1
(KB934268)
Windows XP Windows Media Player 11
Windows XP Update for WMDRM-enabled Media Players (KB902344)
Windows XP Update for Windows XP (KB896344)
Windows XP Windows Genuine Advantage Notification (KB905474)
Office 2003 Update for Outlook Junk Email Filter 2003 (KB943552)
Windows XP Update for Windows XP (KB938828)
Windows XP Update for Windows XP (KB927891)
Windows XP Update for Windows XP (KB930916)

Please help.

This has happened surely because of any one or multiple updates. SO what you can do is you can simply uninstall those recently installed updated one by one until you get the problem fixed. In many cases when all programs refuse to open (like yours as of now), you also wont able to open user accounts in control panel. In such case you log out and log in with default Administrator account and uninstall the updates.

You can also do this by booting into Safe Mode.