Thanks for the advice, I'll try again a few times and if that fails, I'll break out memtest86 and give that a go.
Don't think it's Memory, as I have never had MD5 issues with downloads before. (and my system is currently stable (touch wood) I run XP SP3, not Vista.
Corrupt downloads seem more likely, but weirdly both times I downloaded setup.exe, I got the SAME MD5 hash, and I'd have thought the chances of corruption causing that would be millions to one against, My therry was that either ED have updated the download files and not the MD5 values or somat else is amiss.