How do you fix a system file that has changed because of a virus attack?

We can use the windows feature called SFC(system file checker)
you need the windows CD
#insert the cd
#start the computer in safe mode with command prompt
#to get in to safe mode with command prompt you should keep tapping F8 key after you turn on the computer it will show u the advanced windows optoins, where you need to select safe mode with command prompt,
#when u get the cmd.exe
#type sfc /scannow
#thats it, it will check for corrupted files and repair that