Windows security updates

Can I ask for some advise please?
I have XP sp3 and I can download but am unable to install updates 2656370,4656353 and 2572067, getting error 0x643.

I have tried all sorts of solutions as described on the Microsoft site that seem to have worked for others, run the Fix-it program many times and each time it says it has fixed something but the problem is still there.  I don’t fancy following solutions or links from sites other than Microsoft so am at a loss as to what to do.

I have now hidden the updates because the constant reminders and offer to install at shutdown (which never works) are driving me potty.  Does anyone know if these security updates are essential or will my Norton security software protect me anyway?

Thank you,