First thing you need to do is check the SPN record for that instance of SQL Server. If you do not have access to this, talk to the network folks and have them find the offending SPN record.

If the SPN record is tied to a different account from the account running that instance of SQL Server, then you need to remove the wrong SPN record and add it back in for the correct user.

You can review the following articles for more information:

http://www.sqlservercentral.com/articles/Installation/cannotgeneratesspicontext/929/

http://support.microsoft.com/default.aspx?scid=kb;en-us;811889