I would suspect that the regedit.exe file is corrupt.  There are other possibilities, but that is the simplest one to cure.  Two choices: (1) open Run|msconfig; select expand, type in regedit.exe, and then direct it to the Win XP CD which should be in the appropriate drive.  (2) with the XP CD in the appropriate drive, open a Command Prompt (Start|All Programs|Accessories), type SFC /SCANNOW, then press enter; that will not only restore regedit.exe (if it is corrupt), but also any other corrupted system files it might find.

Bill,

Thanks for useful tip that msconfig can be used to expand files from CD, never noticed that. Had always been doing it from a command prompt, but this is much more convenient. I learn something new every day on this board.

Running a virus scan with the latest definitions would be a good idea, too. There are several viruses that cause regedit problems.

or run it from a command prompt, close the registry, reboot and then try to run it from Start|Run

It won't allow me to run 'msconfig' either...

On my site (Link Below) try the AV section and one of the online virus checkers, such as housecall.

If that does not work try the malware route.
Use these to remove Malware (Spyware and Adware).

1) SpyBot Search and Destroy
After installing SpyBot Search & Destroy, first press Online, and search for, put a check mark at, and install all updates.
Next, close all Internet Explorer windows, hit 'Check for Problems', and have SpyBot remove all it finds.

2) Get Ad-Aware
After installing Ad-Aware, and before running the program, first press “check for updates now".
Click "Connect" and install all updated components available. Click 'Finish'.
Press "Scan Now", then 'next', and let Ad-Aware scan your drives.
It will find a number of "bad" files and registry keys. Click 'Next' again.
Check all found items, and click 'next' once more.
It will ask you whether you'd like to remove all checked items. Click OK.

Always reboot the computer between each program - both of these may find things that they need to have a reboot of the machine to clear - please reboot and let them finish .

Failing those solving your problems a post of a hijackthis log for the experts to advise.
HijackThis From Here
Download, run, scan, save log, then in notepad copy the FULL log by copy and paste as a reply to this post and an expert with HijackThis Knowldge, will have a go at giving advice. Please note the list of experts names below, very few forum regulars here have had this training.

DO NOT FIX ANYTHING WITH HIJACKTHIS WITHOUT EXPERT ADVICE, most of what it finds you need for normal MS Windows tasks.

Known Spyware HijackThis fighters in DellTalk - If you are, and are not on the list please PM Me.
ChrisRLG (Expert in Training LOL, at Tomcoyotes)
YoKenny (Accredited Expert at TomCoyotes)
baskar1234 (Teaching Assistant at TomCoyotes)
Yellowhammer (Expert in Training at TomCoyotes)
TomCoyote (of http://tomcoyote.org/forums/index.php)

You could also go to one of the more specalist forums where more experts will be able to help.
http://tomcoyote.org/forums/index.php
http://forums.spywareinfo.com/index.php
http://www.net-integration.net/cgi-bin/forum/ikonboard.cgi (Home of Spybot S&D)
http://boards.cexx.org/index.php
Do read the sites FAQ before posting, and advise your problem and what steps you have already done to try to cure your problem.

I, and the other hijack experts mentioned above, are in all those sites (and more) with the same login names. You might get one of us at those sites also to anwser your log, but other experts will also be available.