Service Pack 1 installed but update says no

I have an hp Compaq 6516b running Vista Business. I have recently reinstalled the OS and have downloaded and installed all updates. However, Windows Update keeps saying that I need to install Service Pack 1 . I do it and it will tell me installation successful. The recently installed updates file show that service Pack 1 has been successfully installed. Yet, system information says Vista Business with no Service Pack mentioned and updates keeps telling me that Service Pack 1 is ready to be installed.

So why does Vista show that I have successfully installed Service Pack 1, yet keeps asking me to install it? Programs and Features lists Microsoft .NET Framework 3.5 SP1 as being installed on the system.

Thanks