Start by clearing your resolver-cache. (enter the dns-mmc, and clear the server cache).
Then retry nslookup - if the answer is the same, turn on full logging of your dns-server - so that you can trace what is going on with your specific query.
If your server relays the dns query off-host, and the answer is wrong, and you have set up a special forward host for all other domains, throw that hos away - since it has been hit by cache poisoning - and revert back to root hints for proper resolution.
Kind regards / Peter