Apparently he didn't say it:

http://www.wired.com/2012/04/ff_andreessen/2/

====

Anderson: A quote of yours that I’ve always loved is that Netscape would render Windows “a poorly debugged set of device drivers.”

Andreessen: In fairness, you have to give credit for that quote to Bob Metcalfe, the 3Com founder.

Anderson: Oh, it wasn’t you? It’s always attributed to you.

Andreessen: I used to say it, but it was a retweet on my part. [Laughs.] But yes, the idea we had then, which seems obvious today, was to lift the computing off of each user’s device and perform it in the network instead. It’s something I think is inherent in the technology—what some thinkers refer to as the “technological imperative.” It’s as if the technology wants it to happen.