That's strange. The account you first created should have all rights to the box.
XP does the same thing. Files that are created by low level system tasks must often have their permissions tweaked. I have a recurring nightmare with Windows Update files and Executive Software's Undelete telling me that I cannot empty them. I must use a script that sets the permissions appropriately before Undelete allows me to empty them.
This problem is particularly stinky because Windows Update favors using whatever volume has the most space available to make its messes.