If you can even boot Windows 2000 you can boot it in command mode. You have to boot computer from Windows 2000 CD ROM, and then choose option Repair and Recovery console. After this Windows will allow access to all HDD's, FDD's and CDD's, which will help a lot to diagnose a problem.
If you have Windows 2000 Professional installed with automatically installed Service Pack 1 using Windows Update system and you saved files that you downloaded from Internet in Temp folder, sp1express.exe could start download of Service Pack from Internet again after reboot. Problem is in name of folder. You have to name installation folder for SP1by name C:\TEMP\EXT5064.
NTFS file system allows definition of access rights for every file on disk. To grant default rights to system files you can use program MMC. You have to start program by Start/Run/MMC and choose option Console. Choose Add from Add/Remove Snap in and double-click to Security Configuration and Analysis, than click Close and OK buttons. Right click to Security Configuration and Analysis and choose Open Databases. Choose one of options and click Open.
When click to Security Configuration and Analysis in right window you will get path to sdb file. To choose default user rights, choose option Configure computer now, in next dialog box input any name for log file and click OK button.
Rename any program (exe file) to EXPLORER.EXE and copy it to root folder of system partition of Windows (partition C:\), Windows 2000 will start it as shell program, instead of Explorer. You can use for inexperienced users to start directly user software (for example accounting software...).
If Windows 2000 or XP without any reason stop booting reporting problem with system registry file one of reasons could be size of system file. This file is a part of windows registry and it's size is limited to 13 MB. It is larger Windows can not boot. This problem can be seen mostly on computers that are use intense (servers) because of file fragmentation that can increase the file size.
This limitation exists because in a first phase of booting, system use only 16 MB of RAM memory. In that 16 MB have to be loaded NT loader, kernel, HAL and most important drivers. In a normal situations system file takes 4 to 8 MB, but users with a lot of software installed can find this information useful.
Is this a cause of a problem you can find simply by checking of file length. You can find this file in a folder C:\WINNT\SYSTEM32\CONFIG. If you do not want to play on risk back up registry regularly, defragment HDD and use some of registry optimization programs if you notice that system is near to dangerous limit.
If you start more 16-bit applications started under Windows 2000 in a same time, it happens that software simply freeze. Sometimes solution for thi problem can be to install (any) printer and setup it as Default Printer. 16-bit applications sometimes do not work if you do not have printer drivers installed.