Likely your IE configuration.
Did you add the site to Intranet sites yet, per this KB?
http://www.symantec.com/docs/TECH158866
Another suggests Trusted Sites, I've seen both work:
http://www.symantec.com/docs/HOWTO4719
If both of those fail, and the FQDN is being used, and you've verified that it is NOT a DNS issue (believe it or not we encounter a serious DNS issue at about half of our customer sites), then check out this KB, which has you change authentication settings in IIS:
http://www.symantec.com/docs/TECH156006
However, most causes of TECH156006 were resolved in SP2, so it really is a last resort.