Some time if I install an application program on my system, the Task Manager is disabled
What cases it to act like so?

That has happened to me a couple of times before. To make a long story short, it all came down to my computer having a virus from programs i download from torrents sites  or some unreliable source. Check your pc for any virus; i would recommend HiJackthis, and Spybot Search and Destroy. Thats what i used when i had that problem, last time being recently. Now i use a virtual machine to test any program im iffy with before i install it in any of my pc's.