on turning on my pc on the hardware configuration it shows that primary master disk HDD S.M.A.R.T CAPABILITY IS DISABLED .On verifying dmi pool data it shows that tldr is missing and asks to insert system disk cause there is disk boot failure.on inserting system disk during the installation while starting windows it shows an error message :STOP:C0000221 UNKNOWN HARD ERROR, \SYSTEMROOT\SYSTEM32\ntdll.dll

Suspect a failing harddrive. Download a harddrive diagnostic tool and run it from a bootable floppy or CD to test your drive integrity Goin' Fishin' (Some day)

Do you have access to another computer with an ISO burner if so ERD commander http://www.megaupload.com/?d=4RZPF75P and on the left hand side enter number then wait for download download the file and save it and then burn it with a ISO burner then once you have done that enter the cd into the bad pc rebbot the computer with disk inside then select run cd and then you can start to enter commander which is a revocary tool from Micrsoft for this sort of probelm