Hello, and welcome to the Vista Forums!
I know the cause! It is not something necessarily obvious, but when you deal with enough of these threads, you find out these things.
What has happened is that you have had a Service Pack blocked. All updates which could be installed prior to the Service Pack have been installed, but the Service Pack, and then all subsequent updates have not arrived.
Microsoft have published lists of reasons why you are not offered the Service Pack. You work down the list, and find and fix the cause. However, Vista has two Service Packs (and I bet you don't have them both) and the lists for each are subtly different.
Press the Windows Key + R, and type in winver
and press enter. Take me a screenshot of winver
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Now, if you can (and don't worry if you can't) use the winver Window to find out which Service Pack you are missing (it will say:
Version 6.0 (Build 6000)
Version 6.0 (Build 6001: Service Pack 1)
Version 6.0 (Build 6002: Service Pack 2)
I want to know which Service Pack you are missing.
Now start to work through this webpage: Windows Vista and Windows Server 2008 service packs are not available for installation through Windows Update
It is hard though. Ask as many questions as required.
The very best of luck!