How is it for connecting to security sites (avg.com, symantec.com, etc)? If it has a problem (but not to other sites), you've got malware, almost guaranteed.
Also, what is your system like for searching with google, do you get sometimes random redirection to a site that wasn't what you clicked on in the search results? If so, you've got malware, almost guaranteed.
Usually your DNS server should be your router's IP, check that that's the case. If it isn't, and you don't know why, guess what...
