Hi chanlau1,
Please see the following link:
I can't install Microsoft Security Essentials
http://www.microsoft.com/en-lk/security_essentials/Support/cf5220bd-3da8-4694-ac42-f5396ef5ff0b.aspx
By the way, this forum is specially for Exchange server, suggest you seek the solution in the Virus and Malware forum, thanks.
http://answers.microsoft.com/en-us/protect/forum/start-mse1
Frank Wang
TechNet Community Support
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