The SP 3 Install backs up the registry during installation. I went to MS Update and found the download was offered in express download and once it had started I turned off AVG 8 and let it take its course. The windows defender popped up at the later stages and needed permissions which I allowed and all seems well. Took a long while though. Must have been an hour from start to finish. My system is Acer AMD sempron Processor 3100 with 448mb ram.