Are you asking how to do a clean install of Windows 8.1, or how to do a repair install of Windows 8.1?
I am not sure a repair install is even possible if your Setup CD doesn't give you any option to "upgrade" (which would be the repair option). As in a case of upgrading Windows 8.1 to Windows 8.1 (that is typed correctly).
Getting back to Task Manager. Can you replace the file Taskmgr.exe found in C:\Windows\System32 with an identical version of it from another Windows 8.1 machine (copy and paste from a friend)?
If you right-click on Taskmgr.exe and select the "Run As Administrator" option does it work correctly? Do so even if your user is an Admin.
If you test with another Windows user rather than your current user does it work?
Create a user account Windows 8.1
http://windows.microsoft.com/en-au/windows/create-user-account#create-user-account=windows-8
How to perform a clean boot in Windows
http://support.microsoft.com/kb/929135/en-au
How to perform a clean boot to troubleshoot a problem in Windows Vista, Windows 7, or Windows 8
http://support.microsoft.com/default.aspx/kb/929135