I had the same problem, Dell Inspiron 620 Desktop w/ Windows 7 Home Premium. I called Dell tech support, they did a remote access to my computer and attempted the update with the same results. Dell tech said I needed a new motherboard, although all the diagnostics (pre-boot, and through "My Dell Support") do not show any problems. My system most likely would not boot if the motherboard is bad. Dell sent a Dell tech to replace the motherboard, he concluded it was not the motherboard, but some sort of software glitch, ran additional diagnostics, nothing wrong with motherboard. Tech ran install in administrator mode, safe mode, and did a compatibility check, results= not compatible.
Let me know if you have any luck with this.