Windows update problems

On the 31st March, I had a critical update for Windows 7 download, when I switched the computer off it installed the update. The next time I started the computer it went through completing the update, but it then says failure to install update, the coputer then reboots and the same thing keeps happening. I have been able to boot Windows by doing a system restore to a previous date. But the update downloads again, and the same thing happens again.
Can anybody help?

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