Question : Login failed for user 'sa'. (.Net SqlClient Data Provider)

I could not login as sa user in sql server 2005 express edition.
here is the error detail:
Error Number: 18456
Severity: 14
State: 1
Line Number: 65536

I know it is a security issue. but why it is happening?

Answer : Login failed for user 'sa'. (.Net SqlClient Data Provider)

i figured it out after a long days .
i am using kaspersky antivirus software, spyware protection of this software blocks 'sa' access.
i do know how to solve it. simply disable that protection. and i am able to login now.