Are both user accounts "Administrator" and not limited?
Some Viruses can disable Windows components, I'd try running an up to date Virus scan, something like this is OK though a bit limited http://vil.nai.com/vil/stinger/ but free!
If the stinger route did not get you up and going, see if this solution helps you. This little utility will re-enable task manager when some malware(s) out there disable it.
Go to the site from either of these addresses and get a little program/utility which will allow you to enable task manager again. Best to you...