CNET looks legal to me and the file they offer has pretty much the same content as the file paying customers can download from PTC site, including both the 32bit and the 64 bit version in one installation pack. Ofcourse without a valid license (file) its just a 30day demo and after that period it turns into the crippled Express version.
Here is another link where you can download full installations of Prime Demo/Express (32bit and 64 bit sparate) and the site sure is not one of the darker corners of the net:
http://www.adeptscience.co.uk/download/dldcat/33/0/All/Mathcad.html
You may find there other downloads of interest, too.
I don't understand why PTC does not offer that on their site.