There have been a rash of posts about this. Some new malware is
probably causing it. Go through the following malware removal steps
systematically, getting any tools/updates you need from a different,
known-clean machine.
http://www.elephantboycomputers.com/page2.html#Removing_Malware
After you've done this, it would be great if you could report back as
to whether this fixed the issue. R. McCarthy in
microsoft.windowsxp.general has commented that the permissions on the
ENUM key seem to be destroyed by whatever is going through the Windows
community right now. We haven't identified the malware that is doing
it yet.