As per experience and user reviews which I have read till now ,UAC (User Account Control) has been the most annoying feature which got bundled automatically with windows vista.
But some times there are cases when you may want UAC to enabled for one user but at the same time disabled for another user.
So , you can explicitly enable UAC for the specific user by following the procedure below.
1. Open Start>>Control Panel
2. now, double click on user accounts (as shown in the image below)
3. as you can see in the red rectangle in the image above the option to enable / disable UAC per user.
If you have any problem in enabling UAC for a user in vista do let us know through your comments.