Hi Daniel,
In Windows 7 you change the time format in the bottom right corner of the desktop by:
1. On the bottom right desktop clock, right click.
2. In the new window, left click on 'Adjust date/time'.
3. In the 'Date and Time' window, click on 'Change date and time'.
4. In the 'Date and Time Settings' window, click on 'Change calendar settings'.
5. In the 'Time' tab, there are two parameters 'Short time' and 'Long time'
6. Choose the 12 hour or 24 hour format as you wish from the various dropdown options.
That's it!
Regards,
Peter.