Hi!
Are you running a 32 or 64bit Seven?
Is it with every theme or just with one particulary (i had this issue yet...)
Task Manager - Enable or Disable - Vista Forums
Edit : maybe you could try the tool i join...
It is called "Quick Fix" and is supposed to reestablish fonctions disabled by any way (human error, malware,...)
I've never tried it so i can't promise you...
Don't need to install, just double clic (or maybe run as admin...) and choose the fonction to enable.
Please keep us informed...