From my security guide I posted here a while back:
You should always create a recovery disk before messing with system settings. Get a blank diskette. Start -> Programs -> Accessories -> System Tools -> Backup. Select Backup Tab, go to Tools, and select "Create an Emergency Repair Disk".
If for some reason you don't have the Backup program in your system tools (it happens sometimes), you can access it by creating a shortcut. Right-click on the desktop -> New -> Shortcut. In the box type in:
%SystemRoot%\system32\NTBACKUP.EXE
Under the name, type in "Backup", or whatever.
You should update your Emergency Repair Disk (ERD) everytime you install a new program or change important settings. Really. Another reason to create a handy Backup shortcut (I put mine on my desktop), as you can update the ERD more easily. Someday your computer won't start, especially if you are the type that messes around with it a lot (like most of you that are reading this). To use your ERD, you need to boot your computer using the four startup floppies that came with Windows 2000. There is an option that asks if you want to install or repair; choose repair and follow directions. It will ask you to insert your ERD. Many versions of Windows 2000 (mainly OEMs and upgrade disks) don't include the startup floppies. You can make them by running /bootdisk/makeboot.exe off of your Windows 2000 CD. Either create these startup floppies before problems occur, or make sure you have an old DOS boot disk around to access the CD so you can do it later (many Windows 2000 CDs do NOT autoboot, mainly due to hardware issues).
Another handy tool that can be used in the event of an emergency is the Recovery Console. This lets you fix disks when you can't boot into Windows, create/format partitions, and disable/enable services, among other things. It's a useful second step, usually AFTER you try your ERD. While you can run it many ways, the easiest is to load it as a boot-time option (when you press F8 while Windows is loading). To do this, put in your Windows 2000 CD, go to Start -> Run and type in X:\i386\winnt32.exe /cmdcons where "X" is the CD-ROM drive letter (note there is a space between "winnt32.exe" and "/cmdcons"

. I won't go into how to use it here (it's a lot like DOS, and uses some of the same commands). The following article describes it in more detail:
http://support.microsoft.com/support/kb/articles/Q229/7/16.asp