Windows Update

Hi,
I have one question that after updating windows i gets NTOSKRNL.exe error message.What is the reason behind of it.I have checked it on microsoft site and there is a security package 811493 which replace NTOSKRNL.exe file.When i checked it on add/remove programs i do not see any package 811493.Can someone help me to find out the solution of it.
Even if i repair windows it does not solve the problem i have to format the PC.
Regards.
Anurag.