An annoying little program that comes free with many versions of Windows (95 and 98 for sure). It is supposed to check your HDD for errors, but does a poor job of it. Taking entirely too long, it usually only finds such errors as "actual file size different than recorded file size," and does not fix the Windows errors that are actually making your life much more difficult.
It has very few options (probably a good thing). You can choose what drive to scan, whether to do a Standard or Thorough scan, and there is also an advanced options menu.
Advanced options menu: Options regarding summary, log file, cross-linked files, lost file fragments, which files to check for, and MS-DOS mode name length errors.
Thorough options: Options regarding areas of disk to scan, write-testing, and bad sectors in hidden and system files.
Also, when your computer is not properly shut down, upon rebooting, a DOS version of Scan Disk will come up. It, also, does not do you any good, though is supposed to fix whatever caused your computer to not be properly shut down. It can be exited with an 'X' on the keyboard, or by turning its auto-start setting to 'off' with a utility like Tweak-UI or just some very good Regedit skills.
If your disk has not been defragmented in a while, or is too fragmented (I am not sure which), Scan Disk will not run. Unfortunately, the defragmenting tool that comes with Windows will usually not run either at that time, informing you that you must run Scan Disk. Well, unless I have a 3rd party program, I can't do either.
Scan Disk can be found by opening up My Computer (usually on the desktop), right-clicking on any drive, selecting Properties, then selecting the tab "Tools," and clicking the button labeled "Check Now."