Your computer is infected... Anyway do this:
Click Start, Run and type this command exactly as given below: (better - Copy and paste)
REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG_DWORD
You should be able to access task manager now...
And run a full system scan...
P.S. There is no gap in the word 'Policies'...I dont know why its coming... Make sure u remove the spacing....