If you use the tool that krypto mentions on Vista, you'll also need to disable the mcupdate task the task scheduler. Mcupdate will clobber the next Download Time back to its original value.
- Click on the Start Button
- Search for the Task Scheduler – just type Task Scheduler in the search input box at the bottom and press enter
- The Task Scheduler will now be running (after the usual Vista are you sure prompts)
- On the left hand side, drill through the selections Task Scheduler Library / Microsoft- / Windows- / Media Center
- In the middle panel, you will see the tasks, Right click on MCUpdate and choose Disable
Cheers
Mike