I assume you have Windows 9x? You know, since Windows XP doesn't use DOS subsystems and doesn't even have system32\regedit.com. I also assume you have tried normal things like rebooting into safe mode and trying it from there, in which case there's something seriously wrong with your computer.
What I like to do when there's something seriously wrong with my computer is get a new computer. But that costs money, so the other thing I like to do is back up all of my files and then reformat the harddrive, reinstall windows and see if it works then. If it does, then copy all the old files back in. (Don't just copy all the files from your old harddrive, as that will just not work. Only copy the user-created files, like documents, gdp files, etc. Not the actual program files or windows system files)
But I wouldn't do all that just yet. It sounds like there may just be either a permissions problem or a corrupt file or two. You could try copying regedit.com from a friend's computer who has the same OS as you and putting it on your computer and trying again. Or, if you haven't tried it in safe mode and it does work there, then it could be a permissions problem. That I don't know how to fix. But as always, be careful when editing your registry anyway!