For those who do "manual manipulations" there are instructions for how to backup individual subkeys. This probably isn't something you need to be too concerned with.
You are told that you need to "Back up the whole Registry," and there are links in this article to separate detailed instructions for using the "Backup utility" to do a complete backup of the whole "system state," with separate links for WinXP Pro, WinXP Home, and Server 2003. The links (clickies) in the article, which I've disconnected in the paste below, are local links so they only work if you're already at the above page. The "http://…" addresses should work from anywhere.
Back up the whole registry To back up the whole registry, use the Backup utility to back up the system state. The system state includes the registry, the COM+ Class Registration Database, and your boot files. For additional information about using the Backup utility to back up the system state, click the following article numbers to view the articles in the Microsoft Knowledge Base: 308422 (http://support.microsoft.com/kb/308422/) How to use Backup to back up files and folders on your computer in Windows XP 320820 (http://support.microsoft.com/kb/320820/) How to use the Backup utility to back up files and folders in Windows XP Home Edition 326216 (http://support.microsoft.com/kb/326216/) How to use the Backup feature to back up and restore data in Windows Server 2003
The linked articles assume that you want to backup your entire computer, or at least an entire hard drive, so in order to use the Backup utility as the articles describe you need someplace with enough space to hold a whole lot of stuff. This usually means another hard drive or one-or-more CDs. IF you have a place to put it all, this isn't a bad idea, although those with space for the backups can also just "mirror" a drive.
If you don't intend to use the Backup utility on a regular basis, I'll note that I don't use it, and that Microsoft has a "less than sterling" reputation with respect to the "Restore from Backup" function, which no one at Microsoft appears ever to have used.
For a simple backup of only the Registry you can use the instructions given for how to backup individual registry keys:
Click: Start|Run
Type "regedit.exe" (without the quotes) and hit Enter
Make sure that the top level (My Computer) is selected on the left.
Click File|Export
Put an identifiable name for the file in the box (I use date: YYMMDD_Export) and do not change the ".REG" file extension. Choose a place to save the file. Hit Enter.
When finished, click File|Exit. (Note: if you made any changes it should ask if you want to save changes. If you didn't change something on purpose DO NOT SAVE CHANGES.)
For my WinXP the file is on the order of 45 MB or so, so an ordinary floppy probably won't work. I'd suggest making a "REG_Backup" folder on your hard drive and save it there.
Note that a .reg file is effectively an executable file. If you double–click on it, whether on purpose or by accident, all of its content will be written back into your registry. That's what it's supposed to do, but you don't want to do it by accident.
To use the exported .reg file to best advantage, you need to open regedit, open the .reg file in regedit, export the individual keys you need to fix things, and then "execute" only the exported keys.
If you want a backup that you can use to simply "replace everything," you probably should use the full Backup utility process.
Also note that the Regedit export is NOT Microsoft's recommended procedure, although it usually saves enough to get you turned back on if there's a major problem, if you use it carefully.