I, too, have been experiencing this error. Multiple times, and after multiple downloads.
I tried downloading twice via Firefox, and after the second install fail, I commented (politely, although mildly frustrated) about it on Twitter. @ESLightworks replied, and suggested another browser.
Tried Chrome instead. Full download, error on run.
So I also tried most of the steps on
http://nsis.sourceforge.net/NSIS_Error, from the section "If the software was obtained from the Internet", re-downloading almost every time. Some of the steps I couldn't do:
#7 because I tried to download via another computer (using first IE *shudder* and then Firefox), and the download stopped, "complete", at 87MB. I didn't bother trying to copy them across, so I therefore didn't run them.
#8 because I don't have the nVIDIA firewall.
#11 (wasn't necessary).
So, I've downloaded the installer seven or eight times, and every attempt at running it (including one where I ran it as administrator) resulted in the NSIS error. I didn't want to risk the steps in the "Final Options" part of the NSIS page.
My system: Windows 7 Ultimate 64-bit, 2GB RAM, two 140GB hard disks.