XP update problems

On 24 Feb I installed XP updates KB961118, KB967715, the Net Framework
Service Pack 1 and 3.5 Family UPdate [KB951847 x86] and the windows genuine
advantage validation tool KB892130. Now, when I start my computer, it locks
on the Microsoft screen with "chasing" bars. I need to turn it off and
re-start with an error screen that asks me to go back to the last startup
that worked. How do I either [a] fix the problem or [b] uninstall the update
that created the problem.
Thank you.
--
glyn