Well, you can take ownership of the file and then replace it, but if it is a windows file and you replace it with a non-windows file, you might end up in a situation where Vista will claim that "You have made an unauthorized change to your computer..."
Anyways, to change the ownership, open my computer and navigate to c:\windows\system32. Right click on the file you want to replace, select properties. Click on the security tab and then click on the Advanced button. In the new windows that pops up, click on the owner tab. It will list that the current owner is "TrustedInstaller." Click on the "Edit" button below. Another window will show up which will look similar to the previous one. It will list Current Owner: Trusted Installer. From the "Change owner to: " box, select administrators and click Apply. Click OK, to close this window and keep clicking OK until all the windows are closed.
Again, right click the file and select properties. Click the Security tab. Select Administrators from the "Group or user names" list and click "Edit". In the window that comes up, select administrators and then in the "Allow" list in "Permissions for Administrators" check the "Full Control" checkbox.
That should hopefully do it. Just be cautious that if you change a file owned by "TrustedInstaller", your windows installation might run into issues with "You have made an unauthorized change to windows..."