first you need to log on as THE administrator and see if that makes a difference (I doubt it)
it sounds as if a couple of the basic system files have become corrupted - either because it crashed or was switched off at the wrong moment ... or perhaps the disk is on it's way out
or it could be as simple as
http://www.pchell.com/support/taskmanagerdisab led.shtml
if you google the error you'll find hundreds of hits
you could use the install disk to repair
or if you can backup your data a full reinstall is the best option ... but it's not t for the faint hearted
and you need to make sure you have all the drivers for your machine BEFORE you start