Vista - Can't rename a file due to permission problems

I'm trying to rename winhlp32.exe to something else so that I can replace it
with XP's winhlp32.exe.

I'm running as an administrator with UAC enabled and all the default UAC
settings, and I've assigned ownership of winhlp32.exe to myself and granted
myself all permissions, yet when I try to rename or delete it, after
confirming (twice) the UAC prompts, I get a "File Access Denied" dialog box
with a "Try Again" and "Cancel" button. Pressing "Try Again" just brings the
same dialog box back again. Trying from a command prompt running as
administrator gives an "Access is denied" result.

How can I get it to show me what the problem is, i.e. how do I trace its
execution so I can see where the failure is which is leading to the "Try
Again" dialog box rather than successful renaming or deletion?

I'm running a from-scratch install of Vista RC1.