I am an CompTIA A+ Certified IT Technician.
This is a very common problem on many systems, the task manager disabled is a very common virus/malware trick to stop you killing it. The solution for renaming the file is an interesting one, good call. If you want to 'regain' control of your computer, i.e task manager, the quickest way is to reset the registry key that the virus/malware has changed with a .reg file;
Windows Registry Editor Version 5.00
If you copy the above few lines into notepad and most virus or malware don't think anything of this so it is usually possible, then save it to desktop with and ext of .reg - in notepad you will have to save it as type 'all files', then type a 'name.reg' in the box. Once you have saved this file click on it, explorer will ask if you want to add it to the registry, click yes at the prompt and this will 'reset' this key and allow you to access task manager and kill the process(es) and then nuke it with a good anti virus/malware killer. As for preventing re-infection, I always have this reg file on my desktop and scattered around a few places in the computer just in case - And a great FREE anti virus/spyware/firewall/system monitoring software is COMODO Internet Security. Google for it, it is superior to many 'bought' solutions and you can't beat the price.
Hope this helps many people,