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case the application is automatically installed when anyone logs on
to that computer. (And if someone uninstalls the assigned applica-
tion, it is automatically reinstalled at the next logon.)
The Windows Installer is implemented as an operating-system
service in Windows 2000 and is available via service pack for
Windows 95, Windows 98, and Windows NT 4. To use the
Windows Installer, an application must describe itself in an .msi
file. On the .msi file's shortcut menu, you'll find Install, Repair,
and Uninstall options. Thus you can use the shortcut menu to go
directly to an aspect of the program's setup functionality, if you
prefer not to use Control Panel's Add/Remove Programs.
The Windows Installer also has a command-line executable,
Msiexec.exe. Msiexec's elaborate syntax is beyond the scope of
this book. You can read about it by choosing Help from the Start
menu, clicking the Search tab, and searching for msiexec.

Windows File Protection

As mentioned, one of the benefits of the Windows Installer is run-
time resiliency - the ability to repair damaged applications on the
fly. Windows file protection provides comparable resiliency to the
operating system itself. Copies of critical DLLs are maintained in
the super-hidden folder %SystemRoot% \System32\Dllcache. If
any of these protected DLLs is deleted or changed by an
unauthorized agent, a change notification event occurs. If the DLL
was deleted, Windows 2000 supplies a fresh copy from the cache.
If the DLL was overwritten, Windows 2000 checks to see whether
the new copy has a valid digital signature. If it doesn't, the new
copy is overwritten from the cache.
Windows file protection prevents applications (including
Microsoft's) from changing system DLLs. Only service packs and
new versions of the operating system can change critical files.
The System File Checker application (Sfc.exe) that was included
with Windows 98 is still present (for compatibility purposes) in
Windows 2000, but you don't need to use it, because the operating
system looks after the health of your system files automatically.
Nevertheless, if you're curious - or mistrustful of the system's
automatic checking - you can run System File Checker by typing
sfc in a Command Prompt window. Sfc will respond with a list of