Try copying a clean TaskMgr.exe from another pc, its located at c:\windows\system32 . If this doesn't work try these steps:
download http://www.bleepingcomputer.com/files/spyware/KillBox.zip
Extract the program to your desktop and double-click on its folder, then double-click on Killbox.exe to start the program.
In the killbox program, select the Delete on Reboot option.
Copy the file names below to the clipboard by highlighting them and pressing Control-C:
Code:
C:\Program Files\MsConfigs\MsConfigs.exe
C:\WINDOWS\system32\p2pnetwork.exe
C:\WINDOWS\system32\CMD.COM
C:\WINDOWS\system32\netstat.com
C:\WINDOWS\system32\ping.com
C:\WINDOWS\system32\regedit.com
C:\WINDOWS\system32\tasklist.com
C:\WINDOWS\system32\taskkill.com
C:\WINDOWS\system32\taskmgr.com
C:\WINDOWS\system32\tracert.com
Return to Killbox, go to the File menu, and choose "Paste from Clipboard".
Click the red-and-white "Delete File" button. Click "Yes" at the Delete on Reboot prompt. Click "No" at the Pending Operations prompt.
last resort:
Try booting your windows in safe mode, also check your startup programs in msconfig, you can disable some of the virus/trojan there. if you can't use your task manager even in safe mode i suggest you reformat and reinstall your windows and get the latest service pack for your windows operating system. i hope this helps you.