If you are a PC Admin or have admin rights and you are running XP Pro, you
can try running gpedit.msc (Start>run>gpedit.msc). There is a setting in
there, I think, that controls availibility of the Task Manager.
First, you determine that the computer is not infected by a virus
because Task Manager (and Regedit and Cmd) not running is a symptom of
viral infection.
Second, if you've scanned with a current version (not earlier than 2003)
full-featured antivirus using updated definitions - in Safe Mode - and
the computer is clean, you go to MVP Kelly Theriot's website here: