What registry setting is being changed? Is it malware/virus related, you think? If so, that would explain its recurrence. Of course, there could be other reasons as well... an installed program that's checking for that specific setting, and setting it to what it thinks is correct for one.
Try this:
[ol][li]Download
Malwarebytes Antimalware, install, and scan with it to be sure of no malware (not that it's the only program, but if it doesn't find ANYTHING, then more than likely you're clean - no program is 100%, but this one is purty good.)[/li]
[li]Download
Advanced System Care, install it, configure it, stop the initial scan that starts after install, and then hit the big "Care" button, Okaying any prompts. Most likely, you can ignore any other pop-up messages, just close them, including the security advisor. If you take a look and have any thing that sticks out to you from that, search about it, or ask questions about the specific item.[/li]
[li]For Windows XP, You can also try
RegScrubXP - download, install, "program finds problems", then "fix all problems" - it's a really nice piece of software, I just hate they no longer support it.[/li]
[li]Reboot, make sure all is still well, and then.. Turn off system restore.... reboot... turn back on system restore, and set a new restore point.[/li]
[li]Check the registry entry again - see if it was already fixed, or if not, see if you can now fix it. I'm assuming you're 100% certain about what you're doing - otherwise, do some more searching on it to be sure it SHOULD be changed.[/li][/ol]
After all is said and done, report back with your progress.