when i first started having those exact symtoms and thought it may be a virus I did a search for all recent files and found Setup.exe in a folder C:\Program Files\Update06
For some reason i did not have permission do delete the file. I came across this page, found the hijackthis program and ran it ... hijack this came across the file C:\Program Files\Update06\Setup.exe
I immedeately deleted it using the "fix" option.
when i tried Ctrl+Alt+delete the message changed from the regular "This file is being used by another program" to "Task Manager has been disabled by your administrator"
on this site
http://windowsxp.mvps.org/Taskmanager_error.htm I found the fix which was to cut and paste the following in the cmd prompt
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD /d 0 /f
now I had task manager again.
The file Setup.exe was still in its folder C:\Program Files\Update06
I immedeately deleted it and emptied my recycle bin.
Mind you ...
I'm not good at this sort of thing but these are the exact sequence of events that resulted in a solution like five minutes ago.
I did update and run the following programs in this order: adaware, spybot and norton antivrus 2005 then restarted before i did whats stated above. I just thought that I should register and post this in case someone here could figure out what i did right.
Good luck man. I'm going to do a cold boot and see if the solution holds up.