SQL SERVER Authentication problem

I installed sql server 2008 on my computer. i choose SQL SERVER Authentication mode and i give valid user name and password but it gives below error

TITLE: Connect to Server
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Cannot connect to NEAMUL-PC\SQLEXPRESS.
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ADDITIONAL INFORMATION:

Login failed for user 'neamul'. (Microsoft SQL Server, Error: 18456)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&EvtSrc=MSSQLServer&EvtID=18456&LinkId=20476

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BUTTONS:

OK
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