Then it sounds like what you downloaded and or your hard disk and or ram is corrupted, as it's not the file. I don't know what the md5 of it is supposed to be, but I'm sure they'd hash it out, Arcon if you see this, can you start to post that on the site when you upload the new version? So people know they're getting a clean file?
Way to much work for someone as lazy as me. Either you offer to do it for every plugin we push or it's not gonna happen :)
The best way to make sure the file is correct is to download it manually from
http://update.windower.net/plugins/<PluginName>.dll and compare that with the file you got through the Windower launcher.