regedit not opening

This is probably have some corrupted files or corrupted registry entries. Insert your W2K/XP CD. If it loads and displays on the screen click Exit. Then go to Start, Run, type sfc /scannow ,click OK. The program will check for corrupted files and reinstall them.
Unfortunately it does not provide a report.

If that does not work then your next step would be a repair using the Win2K/XP CD but you will need to change the booting order of your computer beforehand. When you start up your computer, press the Delete key (sometimes it is F2 or F5) for the BIOS screen to appear. There you will be able to change the booting sequence to CDROM being first. Save and reboot. During the boot you will need to press a key so that it boots of the W2K CD. When choices appear as Install, Repair etc. Choose Repair, then Fast. When finished allow it to reboot to Windows and see if your problem has been resolved. If it has been resolved then, while rebooting, change the sequence back to the original one, save and proceed.

If it hasn't been resolved then you could repeat the process using Manual repair. This process, however, reinstalls all original files and you will need to update after the process. Click "Yes to All" to replace the existing files with the original ones from the CD. This process will not overwrite your Data files. Windows will now work but you will need to go through the update process again. If you haven't got the SP4/SP2 update disk then get one from MS, it's free.