Can't Start SQL ServerAgent

I installed SQL2005 (first time). I have a couple of problems that perplex me.

I can connect to my instance when starting up SQL Management Studio no problem. After connecting, I can manually backup and restore DBs, no problem.

However, when I attempt to start SQL Server Agent - I get the follwoing message:

Unable to start service SQLAgent$GREATPLAINSV80 on server USIND-BAT3835N1.

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The SQLAgent$GREATPLAINSV80 service on USIND-BAT3835N1 started and then stopped. (ObjectExplorer)

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Program Location:

at Microsoft.SqlServer.Management.UI.VSIntegration.ObjectExplorer.Service.Start()

The error log is as follows:

Date,Source,Severity,Message
05/13/2006 12:08:57,,Information,[098] SQLServerAgent terminated (normally)
05/13/2006 12:08:56,,Error,[382] Logon to server 'USIND-BAT3835N1GREATPLAINSV80' failed (DisableAgentXPs)
05/13/2006 12:08:56,,Error,[298] SQLServer Error: 772<c/> Cannot generate SSPI context [SQLSTATE HY000]
05/13/2006 12:08:56,,Error,[298] SQLServer Error: 772<c/> SQL Network Interfaces: The Local Security Authority cannot be contacted [SQLSTATE HY000]
05/13/2006 12:08:56,,Error,[000] Unable to connect to server 'USIND-BAT3835N1GREATPLAINSV80'; SQLServerAgent cannot start
05/13/2006 12:08:56,,Error,[298] SQLServer Error: 772<c/> Cannot generate SSPI context [SQLSTATE HY000]
05/13/2006 12:08:56,,Error,[298] SQLServer Error: 772<c/> SQL Network Interfaces: The Local Security Authority cannot be contacted [SQLSTATE HY000]

I researched using Microsoft SQL Home knowledge base for error 772, cannot generate SSPI context and [SQLSTATE HY000] and didn't find anything that seems to apply.

Why can I connect to the database engine but not start SQL ServerAgent?