Unable to open Task Manager/ Mozilla firefox/IE/Google Chrome/iTunes

System Configuration :

Windows 7

Norton Internet Security (Ran live Update  today (after six months ))

RAM : 4GB

Hard Disk : 500 GB

 

Issues encountered :

- Unable to open Task Manager/ Mozilla firefox/IE/Google Chrome/iTunes

- Unable to  Uninstall any Software

- My computer is loading very slowly

- After clicking Right click, options are getting displayed after 10 mins

 

Ran Local Disk Scan - Scanned full C:\ - Application is getting stalled for 1 hour with the status processing 2 files.

NIS displays system is secure.

 

Do i have to format the whole system and re-intsall all the applications again?