Recovery file system is missing / damaged

If once the windows file system (such as *. dll and *. sys) which is broken (corrupt) or lost, so that windows can not booting (only display a message that the file xx.dll was mising or corrupt), then try to follow these steps:

1. Restart the computer from the Windows Setup Floppy disk or the Windows Installation CD.
2. When the show "Welcome to Setup" screen press F10 or press R (Windows 2000 User must press F10 or R and C).
3. When shown a list of installation and the text "Which Windows Installation would you like to log on to?", Select the appropriate number according to the Windows installation that you want to repair.
4. Write the Administrator Password and press Enter. If no password, press enter directly.
5. Of the view that appears, type the command as follows:
   expand D:\i386\xx.dll C:\dir_target
   Where xx.dll name of the file is damaged / lost to in the recovery, D: is the drive letter for the CD-ROM and dir_target is direktory target, and then press enter.
6. After that type in exit to exit the CD and remove it and restart the computer

Reference:
http://antivirus.about.com/od/windowsbasics/ht/windowsrecovery.htm