Securing the cache against pollution helps to prevent spoofed DNS records from polluting the cache. This is an important setting on external DNS servers and I even recommend it for internal DNS servers.

Securing your DNS servers from cache pollution is rather straight-forward. Open up Administrative Tools, and then click DNS to open the DNS Management MMC. Right-click on the DNS server and select Properties, then select the Advanced tab. Check the box next to Secure Cache Against Pollution.

That's all there is to it! Now Windows will still trust zone transfers from BIND servers so if you are using BIND you should upgrade to BIND 9 which automatically scrubs poisoned records.