I’ve had issues with a Vista workstation where randomly some sites will become unavailable. The only way I could get access back to the site would be to flush the DNS cache. Taking a look at the DNS cache indicated that for the sites that weren’t resolving, they had a status of “Server failure of Type A”.
Doing a little research on this issue, I came across a knowledge base article by Microsoft. The KB article (928233) says that the issue is a result in the way Vista handles the DHCP Broadcast flag in discovery packets. XP had this disabled by default while Vista does not. If the router that you are using does not support the broadcast flag it can cause corrupt DNS cache entries on your Vista machine. The solution to the problem is to disable the broadcast flag.
To disable the flag you need to use the registry editor to make changes under this subkey, where GUID corresponds to your network card:
Create a new DWORD (32-bit) Value with the name “DhcpConnEnableBcastFlagToggle” and set it’s value to “1″.