There are a lot of a different things that can cause network latency. The first step would be to try and determine if it is a problem with that particular machine, which it sounds like you have been trying to do. When you say you tested with another computer, was everything else the same? For example, was it connected to the same wireless access point with comparable signal strength on the same channel, etc? If it turns out to be related to the computer, you could have a setting mismatch, for example duplex being set to auto on one end and forced to 100 on the other. Another possibility is a routing issue, where sometimes packets can get caught in a routing loop before finally going towards their destination. This is more prone to happen in more complex business networks than simple home LANs.
One tool that might help to determine exactly where the latency is coming from would be pathping (Windows) or Traceroute (Linux). It will show you the route between hosts, such as your PC and google, as well as tell you the time to each step. This will show you if your traffic is getting lost and where the major latency is occurring.
One other thing that just occurred to me is that especially on the first lookup, you could be facing a high DNS resolution time. Try pinging via IP address instead of name to eliminate this possibility.