Unable to run Many important applications

I am unable to run regedit, task manager and many other imp. Softwares in my Windows 7 Laptop(Dell E5400). Since i did some changes and deleted some viral files, i think these problems were created since then. Many good methods faied to re-enable regedit and task manager. I am unable to open Adobe Reader, Audio Video Players, Systems hangs when i try to uninstall any application. Pls Help, Its Urgent to Solve.