If you have applied Service Pack 4 or 5 more than once, running a repair may generate the following message
The following value in the Setup Information File is corrupt or missing: Value XXX on the line in section XXX with key XXX. Setup was unable to copy the file Certmgr.hlp. To skip this file, press ESC. WARNING: this option is intended for advanced users who understand the ramifications of the absence of the various Windows NT files. If you choose to skip this file, Setup cannot guarantee successful installation of Windows NT. To exit Setup, press F3.The problem is caused because the Setup.log file had the wrong checksum values.
You can safely skip these files.
To fix the problem, upgrade to SP6.