Task Cheduler Access is denied

Hi Guys!

I am facing this problem on our network and tried to Google and Bing for it
but to no joy.
Basically whenever I try to run a task Schedule I get this error message
"0x80070005 Access is denied" I have checked all the permissions I could
think and all is fine but I cannot understand why I am getting this message
since I got full administrative access to the computer. I downloaded
KB904423-x86 from TechNet but this would not apply to my service pack since I
am running XP SP3.
Can anyone please help me solve this?