why cant i update critical updates?

i can not install critical windows updates, necessary
to the health of my computer. (vista)
i have not been able to do so for some time. (feb 17-08)
i am starting to notice changes in the response of my system.
i have tried to do every thing i know how to do, to diagnose
my problem. what can i do to get more information?
after the updates fail, i get the error code: 800b0100
can anyone out there help?-chris model no#CNH73602GT