The problem isn't on your end, it's on Cnet's. There are a number of things they could be doing, but it's their fault, not yours.
I use IE7 on windows xp, when reading online courses I click on 'next lesson' or 'go to classroom' I recieve message on webpage Error 404 page not found.
I've turned off pop-up blocker whilst on cnet.
Bit silly but pse help