Destination host unreachable: The host that you are trying to ping is down or is not operating on the network.
Request timed out: The ping command timed out because there was no reply from the host. Ping your local address 127.0.0.1. This will tell you that TCP is functioning. It does not tell you that your network card is functioning. Ping your default gateway or next hop Router. This will show that the router is up. Ping beyond the next hop Router. If the ping fails, the request timed out can mean that the destination host is down or that there is no route back to you. Destination Net Unreachable will show the IP address of the Router that tried to route the packet but did not have a valid route.
As you can see, there is a big difference between "destination host unreachable" and "request timed out." This is particularly noticeable in Windows Vista, where ping's error messages have been tuned to be more specifically accurate. As such, a bad response in Vista has a tendency to get a different answer than the usual Windows "request timed out."