Corrupted system files can make Windows or applications behave badly or fail completely. A crucial program can be damaged by a system crash, for example, but you don't know that it's happened – just that Windows isn't working properly any more.

System File Checker will scan the important software components and report ones that aren't as they should be. You then get the option to restore damaged files. The program is set by default to scan certain files supplied with Windows 98. You can configure it to take in other files, such as those belonging to applications you have installed.

When you run System File Checker you see this window:

Just click Start to scan the file set already defined. Should a problem be found, this is an example of the error report:

You can choose to ignore the warning, replace the file from the installation disks, or tell System File Checker that the current state of this file is correct and should not trigger a warning in future. You would do this if you knew the file had been updated, and that SFC had simply detected the new software.

If you want SFC to keep an eye on applications, press the Settings button in the main dialog. This brings up a further dialog on which there is a Search Criteria tab:

You can add new folders to the search list, or change the status of existing ones. Here, for instance, the default state of Program Files is for subfolders not to be searched. Highlighting Program Files and clicking on Include Subfolders will switch the Subfolders column setting to 'Yes'.

Now perform a scan, so that the current state of files can be recorded. The log produced by this process will be referred to in future checks. By default, the log file is \WINDOWS\DEFAULT.SFC. If you want to change this, you can do so on the Advanced tab.

For a report on what SFC has done, click View Log on the Settings tab:

This will display the file that logs SFC's activity (this is not the same log as the record of each scanned file).

On this tab you can also say what should happen to files that are about to be replaced with original versions. The default is for SFC to ask you whether you want to back up the file about to be overwritten. The other options are to always back up automatically or never to back up. Neither of these options causes prompts to be displayed.

The bottom options here specify whether or not SFC should tell you about files that have been deleted, and ones that have been changed. Note that not all files in the named folders will be checked – only those of the types listed on the Search Criteria tab.