There is nothing wrong.
Maxtor calls this a "160 GB drive" because they define "1 GB" as exactly 1,000,000,000 bytes. BUT Windows defines "1 GB" to be 1024 x 1024 x 1024 bytes, or 1,073,741,824 bytes. Using that latter way of counting, 160,000,000,000 bytes comes out to 149.01 "GB".
You are NOT missing any space. When Windows counts up file sizes, Free Space, etc., it uses that same counting system consistently. So it will tell you that a large file is slightly smaller than a Maxtor salesman would tell you. They are telling you the same thing, but they confuse everybody by using two different definitions of the term "GB" and quoting different answers.