try to d/l the files again...
eventually try the secondary source. compare the file sizes. Then run the assembler again. If you get the .exe file it sohould usually be fine.
Edit: oh, and there are md5 sums on the bottom of the page. so the easiest thing to verify the downloads is to check the md5 sums.
(dunno how that works on win, but afaik its described somewhere in the forums)