from :
http://support.microsoft.com/kb/948537
To uninstall Windows Vista SP1 by using the Programs and Features item in Control Panel, follow these steps:
1.Click
Start
, type Appwiz.cpl in the
Start Search box, and then click
Appwiz.cpl in the
Programs list.

If you are prompted for an administrator password or for a confirmation, type the password, or click Continue.
2.Click
View installed updates.
3.Click
Service Pack for Microsoft Windows (KB936330), and then click
Uninstall.
4.Follow the instructions that appear on the screen to complete the uninstall process.
Method 2 :
Important Before you use System Restore, make sure that you have restarted the computer at least one time since you have installed Windows Vista SP1. By restarting the computer, you allow for any remaining servicing processes to finish.
1.Click
Start
, type rstrui in the
Start Search box, and then click
rstrui in the
Programs list.

If you are prompted for an administrator password or for a confirmation, type the password, or click Continue.
2.In the Restore System Files and Settings window, click
Next.
3.In the Choose a Restore Point window, click
Install: Windows Vista Service Pack 1, and then click
Next. Or, click a restore point that was created before you installed Windows Vista SP1, and then click
Next.
Note The "Install: Windows Vista Service Pack 1" restore point is the restore point that was created immediately before you installed Windows Vista SP1.4.Verify the restore point that you want to use, and then click
Finish.