Run sfc.exe and select the "extract one file..." option.  Type in the
complete name of the missing file you want to restore or use the browse
button for a file replacement.  Click start and in the resulting window,
browse to the location of the folder containing the W98 .cab files (either
the hard drive or CDRom drive).  If the information in the "save in" box is
wrong, use the browse button to show where the file is to be restored to.
Click OK and if appropriate, on the resulting screen, show the location
where the file being replaced is to be backed up to.  Click OK or skip as
appropriate.  Restart your computer and the file replacement will occur
during the boot process.

If you are going to use sfc.exe for other than the extract option, I highly
recommend that you first read the article here:
http://home.satx.rr.com/badour/html/using_sfc.html