Well, a compromised google.com main page could return malicious search results for certain queries. How many Windows sysadmins install PuTTY by googling "putty", and then installing an executable from whatever site shows up in the first couple of results?...
If the primary install method is "search and download whatever manually from the internet," you have bigger issues than a potential Google compromise: create a site with better ranking than the canonical HTTP (!) download page, MITM the HTTP download, whatever.
