The sidebar on Windows Vista can be a useful new tool for your desktop, but if you wanted to know how to add a clock to you sidebar then just follow this;

• Right click in your sidebar
• Select add gadgets
• Select the clock gadget
• Done

You can also edit and personalize your clock by doing this;

• Hover over your clock and select the edit button
• Here you can change these
  • Clock design
  • Clock name
  • Show the second hand
  • It also gives you the option to change the time (note: this is the time your whole computer runs on)