The fastest way to lock your PC
 

You used this shortcut when you still had Windows XP. Now, you can still do it with Windows Vista. Here are the steps to do it.

1. Make a right click on an empty space on the Desktop.
2. Choose New on the pop-up window, and then Shortcut.
3. Enter the words “rundll32.exe user32.dll, LockWorkStation” exactly, when prompted to type the location of the item on the space provided for.
4. Click on Next.
5. Enter a name for your shortcut, like Lock Shortcut or something more ingenious.
6. Click on Finish. Then you are all done. Wasn’t it fast?

OR if you don't have user switching enabled, try WINKEY + L

It does the trick too