registry editor won't open

Are you logged in as Administrator, what OS? whats the error message?

Check for virus.

best bet is to scan for a virus, then put in your windows disk, go to start -- run -- type
sfc.exe /scannow

Or you can try to copy regedit from another similar system

This can happen when your system becomes infected with certain viruses. Rename regedit.exe to regedit.com and try running it again. Should this solve your problem then a virus has hooked the .exe files and blocks regedit and msconfig from running. Msconfig.exe can also be renamed to msconfig.com and allows it to run as well. You can then work on identifying and removing the virus.

I hope this helps you.