i test all wim archives using 7zip. Install.wim is ok, but boot.wim is damaged.
I tested a known good Win8 Dev Preview install disk with Win7Zip and it also reported the boot.wim as being damaged. But this disc worked fine for several Win8/32bit installs.
Where did you get your Win8 .ISO from?
Did you run a checksum test to see if the .ISO file downloaded correctly?
The SHA-1 checksum hash data is located right on the official MS download siteMD5 & SHA-1 Checksum Utility
If the checksum of your download matches the checksum on the MS download site you know the .ISO file downloaded correctly.
And that might be pointing at a bad disc or a bad burn to the disc being at fault.