Who blocks RUN registry keys?

Hello.

I tried to install USBSafelyRemove program and have found that it failed to install the key in HKLM\Software\Microsoft\Windows\CurrentVersion\Run.
I tried to do this manually, but also failed (sorry, don’t remember the message exactly, something like “Registry blocked”). In other registry branches I can create keys without problems.
I have installed Avast Internet Security and Avast Cleanup - do they block registry?
If yes - how to allow the key creation?

Thank you.
Konstantin

PS. Windows XP. Yes, I have admin rights