This is a Vista/Win7 problem.  In a nutshell, Microsoft doesn't trust you any further than it can throw you, so most of the system files can't be altered, even by the (ha-ha) 'Administrator'.  This even includes such innocuous files as the sound files in the 'Media' folder.

To get around this snag is a semi-elaborate two-step process:

• Right-click the folder the to-be-changed files are in and open Properties.
   
• Click on the 'Security' tab, then the 'Advanced' button.
   
• Click on the 'Owner' tab, then 'Edit'.  Highlight your 'user name/computer name' entry, check the box next to 'Replace owner…' down below, click OK.  You may get a few error boxes popping up on you if there are some 'in-use' files that simply can't be changed.
   
• Close the Property boxes.
   
• Again right-click the folder and open Properties.  Click on the 'Security' tab.
   
• Click on the 'Edit' button and select your 'user name/computer name' entry.  Down below, click on the first box under 'Allow' and the rest will auto-check.
   
• OK back to the folder and the files should be able to be altered.

How simple does it get?