After a visit to google, I'm not so sure this name is worth just the reg fee.
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There is a "Guard Security" Bank
http://www.guardsecuritybank.com/ that might be interested. They appear to own "Guard.com" and are the type to buy up everything remotely similiar to their "business name"---Guard Security.
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Technically "Security Guard" is the best conjugation of "Security" and "Guard", however people do use "Guard Security"... As in "Top Guard Security" at XYZ event.
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This name would best be used for a specific product or business. What a better name for a security company: Guard Security... (Security Guard Co. is too obvious is why you don't see too many of them with the "obvious" name).
Good luck! As with most names, beauty is in the eye of the beholder.... I see a possibility here.