My security settings will not load correctly at startup

Could someone please help with the following:

Each time I start my system for the first time and try to access a program such as the task manager, I get an error message "C:\WINDOWS\system32\taskmgr.exe cannot be found. You may not have the correct permissions to access this file"

Considering that I am the system administrator, this is confusing.

I normally get around this problem by switching off without logging off and then restarting. Then everything works fine.
However I have just installed the latest windows xp update and I had to restart eight times before I had access to the programs that I should have access to.

I have the following system specs:

Windows xp SP2 with the latest updates installed
Pentium 4
512 MB ram
60GB hard drive

Virus checking software : Norton internet security 2004 and Panda Truprevent
Spyware checking software : Spysweeper, adaware se, spybot and others

Many thanks in advance