that's normal.
HDD manufacturer: 320GB = 320,000,000,000 bytes (giga- is a prefix meaning billion, so 320 billion bytes = 320 giga-bytes)
OS: 320GB = 343,597,383,680 bytes (1024KB is 1MB, 1024MB is 1 GB)
So while it's not technically false that the drive is 320GB, it's not going to be as large as you expect it to be
sorry for the math but that's what it is