Account is currently locked out issue

Hi experts,

 

I have a starnge problem in sqlserver 2008 R2.I have installed sqlserver and connecting using sqlserver authentication.It is working perfectly few days.But after some days the connection cannot established and reporting error.The error showing that 'Account is currently locked out.Your system administrator can unlock it".What might be the reason of this locking.If i unlock it,then it will work fine.But after some time sql server is again locked.

 

What is the exact problem?Is the problem with the installation ?

 

Please help me