option 1
insert your windows disk into your optical drive. next open the "run"
command in the start menu. type in "sfc" (without the quotes
obviously) and press return or run. windows will scan your system
files for problems and rewrite new files over the corrupted ones.
option 2
start your computer with the windows disk in the drive and select the
repair option. windows will write a new version of itself over your
existing installation leaving your personal files mostly intact.
option 3
start your computer with the disk in the drive. select the option to
install a new copy of windows onto your computer. choose the option to
delete your existing copy of windows and write a new version onto your
hard drive. CAUTION: THIS WILL DELETE EVERYTHING ON YOUR COMPUTER AND
WRITE A FRESH NEW COPY OF WINDOWS WITH NO FILES OR APPLICATIONS
(beyond what is already included in windows)