When you install a Service Pack for Windows 2000, you have the option of keeping a backup copy of all the files that are replaced by the Service Pack. This is generally considered to be a good idea; sometimes applying a Service Pack can cause unexpected problems, and might need to uninstall the pack to continue normal operations. If you don't keep the replaced files, the only other way to do this is to restore the whole system from a backup.
A server that has been in place for a long time may accumulate a great many backup folders from the installation of Service Packs and pre-SP hotfixes. If the server has been running dependably for quite some time (usually a month or two is enough to give a Service Pack a good shakedown), you can usually remove the Service Pack uninstall files and free up some space (around 25-100 MB per Service Pack).
Service Pack backup files are kept in a hidden folder in the \%Systemroot% directory, named $NtServicePackUninstall$ . You need to have Explorer show hidden and system files to be able to see it; you can enable this in Explorer by selecting Tools | Folder Options | View | Show hidden files and folders. Once you see this folder, simply remove it by deleting it.
There will also be an entry to uninstall the Service Pack in Add/Remove Programs. To remove this entry, navigate in the Registry to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall\Windows 2000 Service Pack
Note that there is another folder in \%Systemroot% named ServicePackFiles. Do not delete it. If a file installed from the Service Pack becomes corrupted, Windows File Protection will attempt to load restored versions of the files in question from that folder. If you've removed the folder, WFP will fail -- it won't ask for an alternate location, but will simply not replace the corrupted file.
Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter.