sql server 2008 enable sa account

I logged into my PC with administrator account. When I try to enable the 'sa' account of SQL Server 2008, I get the following error:

TITLE: Microsoft SQL Server Management Studio
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Alter failed for Login 'sa'.  (Microsoft.SqlServer.Smo)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.50.1600.1+((KJ_RTM).100402-1540+)&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Alter+Login&LinkId=20476

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ADDITIONAL INFORMATION:

An exception occurred while execut开发者_C百科ing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)

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Cannot set a credential for principal 'sa'. (Microsoft SQL Server, Error: 15535)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=09.00.1399&EvtSrc=MSSQLServer&EvtID=15535&LinkId=20476

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BUTTONS:

OK
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Have you tried Microsoft KB956177 before?

To resolve this issue, make sure that you select the Map To Credential check box on the General tab of the Login Properties - sa dialog box before you change the "sa" login properties and then click OK to close the dialog box.

To resolve this issue, make sure that you select the Map To Credential check box on the General tab of the Login Properties - sa dialog box before you change the "sa" login properties and then click OK to close the dialog box.