The difference is hard drive marketing.
Unfortunately, the 160GB (and 320GB) numbers represent billions of bytes... not true gigabytes.
According to drive manufacturers: 320GB = 320,000,000,000 bytes.
However, in the real world, 320,000,000,000 is really 298GB (binary-base2)
So, nothing is wrong with your system, just a little sales fraud going on.
You haven't lost any capacity - the two numbers are actually equal.