Do you mean an emergency repair disk? In Windows NT that's created with the command:

RDISK /S

The /S parameter adds the SAM or SECURITY hive to the others (I forgot which).

In Windows 2000 it is created with NTBACKUP.EXE. In Windows XP the function is no longer there. You can only create a backup of the system state with NTBACKUP.EXE on some other, bigger media.

Or do you mean a boot diskette, from which you can boot the computer if anything goes wrong with the boot files? For that you have to format a diskette from Windows NT, 2000, or XP (it doesn't work from the lesser versions of Windows), then copy the following 3 to 5 files from the root folder to the diskette:

1. NTDETECT.COM
2. NTLDR
3. BOOT.INI
4. BOOTSECT.DOS (if present)
5. NTBOOTDD.SYS (if present)

Some of these files have the hidden or system attributes, so you may not always see them. These attributes are not required though. You can remove them temporarily (using ATTRIB or Explorer) and you don't need them on the diskette.

Then try to boot from that diskette. If the computer boots as usual, the boot diskette is OK.

Yet another kind of boot diskette is the diskette set you need to install Windows in case the computer cannot boot from the CD-ROM drive. A program to create these is on the install CD for Windows 2000 and XP. More information for XP can be found in the following Microsoft Knowledge Base article:

Obtaining Windows XP Setup Boot Disks
http://support.microsoft.com/kb/310994/