<pedantry alert> There is no DOS in Windows XP. There is, however, a Command Prompt from which you can use many of the commands that used to be available in DOS. </pedantry alert>
Using the "copy" command is not the way to backup your drive. Copy is used for copying files to a specified location. So unless you want all your files in one giant directory, it's not much use as a backup.
The "xcopy" command copies files and directories, including subdirectories.
Although I haven't used it, robocopy is a much better tool than either copy or xcopy. In fact, it's name derives not from "robot copy" as you might assume but from "robust copy." Robocopy can be obtained
as part of the Windows Server 2003 Resource kit.
None of these copying tools is particularly designed as a backup tool. You would be far better off getting disk imaging or disk cloning software. These applications are not free, but they make restoring far easier. Here are a few:
Acronis True Image Home
Paragon Backup and Recovery
Casper 6.0 Backup